**HydroSHEDS**

**Technical Documentation**

**Version 1.0**

**Bernhard Lehner**

**Conservation Science Program, World Wildlife Fund US, Washington, DC
20037**

**Kris Verdin**

**USGS Earth Resources Observation and Science, Sioux Falls, SD 57198**

**Andy Jarvis**

**International Centre for Tropical Agriculture (CIAT), AA6713, Cali,
Colombia**

**May 2006**

1\. Overview

2\. Data sources

3\. Data set development

4\. Quality assessment

5\. Data layers and availability

6\. Data formats and distribution

7\. Notes for HydroSHEDS users

8\. References

9\. Disclaimer

Appendix A: Flowchart of the generation of HydroSHEDS

**1. Overview**

HydroSHEDS (**[Hydro]{.underline}**logical data and maps based on
**[SH]{.underline}**uttle **[E]{.underline}**levation
**[D]{.underline}**erivatives at multiple **[S]{.underline}**cales)
provides hydrographic information in a consistent and comprehensive
format for regional and global-scale applications. HydroSHEDS offers a
suite of geo-referenced data sets (vector and raster), including stream
networks, watershed boundaries, drainage directions, and ancillary data
layers such as flow accumulations, distances, and river topology
information.

HydroSHEDS is derived from elevation data of the Shuttle Radar
Topography Mission (SRTM) at 3 arc-second resolution. The original SRTM
data have been hydrologically conditioned using a sequence of automated
procedures. Existing methods of data improvement and newly developed
algorithms have been applied, including void-filling, filtering, stream
burning, and upscaling techniques. Manual corrections were made where
necessary. Preliminary quality assessments indicate that the accuracy of
HydroSHEDS significantly exceeds that of existing global watershed and
river maps.

The goal of developing HydroSHEDS was to generate key data layers to
support regional and global watershed analyses, hydrological modeling,
and freshwater conservation planning at a quality, resolution and extent
that had previously been unachievable. Available resolutions range from
3 arc-second (approx. 90 meters at the equator) to 5 minute (approx. 10
km at the equator) with seamless near-global extent.

HydroSHEDS has been developed by the Conservation Science Program of
World Wildlife Fund (WWF), in partnership with the U.S. Geological
Survey (USGS), the International Centre for Tropical Agriculture (CIAT),
The Nature Conservancy (TNC), and the Center for Environmental Systems
Research (CESR) of the University of Kassel, Germany. Major funding for
this project was provided to WWF by JohnsonDiversey, Inc.

HydroSHEDS data are free for non-commercial use.

For more information on HydroSHEDS please visit

> <http://www.worldwildlife.org/hydrosheds> (homepage) and
> [http://hydrosheds.cr.usgs.gov](http://hydrosheds.cr.usgs.gov/) (data
> download and technical information).

Constructive comments from users of HydroSHEDS are welcomed. Please send
your comments to <hydrosheds@wwfus.org>. Please be aware that we may not
be able to reply to individual requests due to limited capacities. We
will regularly update the technical documentation to address key
questions and topics.

**2. Data sources**

This section briefly describes the main data sources that have been used
in the generation of HydroSHEDS. The actual processing steps are
addressed in section 3. Please also refer to the flowchart of Appendix
A.

2.1 Shuttle Radar Topography Mission

The primary data source of HydroSHEDS is the digital elevation model
(DEM) of the Shuttle Radar Topography Mission. SRTM elevation data were
obtained by a specially modified radar system that flew onboard the
Space Shuttle Endeavor during an 11-day mission in February of 2000. The
SRTM project is a collaborative effort by the National Aeronautics and
Space Administration (NASA), the National Geospatial-Intelligence Agency
of the U.S. Department of Defense (NGA), as well as the German Aerospace
Center (DLR) and the Italian Space Agency (ASI). NASA's Jet Propulsion
Laboratory (JPL) managed the mission and the Earth Resources Observation
and Science Data Center of the U.S. Geological Survey (USGS EROS Data
Center) has the responsibility of hosting, distributing and archiving
the final SRTM data products. A general description of the SRTM mission
can be found in Farr and Kobrick (2000).

2.2 SRTM elevation data, Version 1 (SRTM-1 and SRTM-3 unfinished data)

The raw SRTM data have been processed into an initial research quality
DEM by JPL. No further editing has been performed, resulting in a data
set that may contain numerous voids and other spurious points such as
anomalously high (spike) or low (well) values. Since water surfaces
produce very low radar backscatter, water bodies are generally not well
defined and appear quite "noisy". Coastlines, as well, are not clearly
defined. For areas outside of the conterminous United States (CONUS),
the original 1 arc-second data (SRTM-1; cell size approximately 30
meters at the equator) were aggregated into 3 arc-second data (SRTM-3)
by averaging, i.e. each 3 arc-second data point is generated by
averaging the corresponding 3x3 kernel of the 1 arc-second data. For
more details see NASA/JPL (2005).

2.3 SRTM elevation data, Version 2 (DTED-2 and DTED-1 finished data)

After JPL completed the raw processing of the SRTM-1 and SRTM-3 data,
NGA performed quality assurance checks and then carried out several
additional finishing steps to comply with the required data standards of
the Digital Terrain Elevation Data (DTED®) format (NASA 2003). Spikes
and wells in the data were detected and voided out. Small voids were
filled by interpolation of surrounding elevations. Large voids, however,
were left in the data. The ocean was set to an elevation of 0 meters.
Lakes of 600 meters or more in length were flattened and set to a
constant height. Rivers of more than 183 meters in width were delineated
and monotonically stepped down in height. Islands were depicted if they
had a major axis exceeding 300 meters or the relief was greater than 15
meters. All finishing steps were performed at the original 1 arc-second
resolution, resulting in DTED Level 2 data products. DTED-2 was then
aggregated into 3 arc-second DTED-1 data. Unlike SRTM-3, however, DTED-1
has been generated by subsampling, i.e. each 3 arc-second data point is
generated by assigning the value of the center pixel of the
corresponding 3x3 kernel of the 1 arc-second data. For more details see
NASA/JPL (2005).

2.4 SRTM tiling format and data availability

SRTM elevation data have been processed in a systematic fashion and
mosaicked into approximately 15,000 one-degree by one-degree tiles.
Following the DTED convention, the names of the individual data tiles
refer to the latitude and longitude of the lower-left (southwest) corner
of the tile. For example, the coordinates of the center of the
lower-left pixel of tile n40w118 are 40 degrees north latitude and 118
degrees west longitude. In the case of DTED-1 and SRTM-3 data, a single
tile consists of 1201 data rows and 1201 data columns. Due to the
definition via pixel centers, the four edges of a tile each exceed the
assigned coordinates by half a pixel and the outermost rows and columns
of adjacent tiles are overlapping. For more details see NASA/JPL (2005).

Outside of the CONUS, the 1 arc-second products (SRTM-1 and DTED-2) are
only available upon request for scientific purposes. The 3 arc-second
products (SRTM-3 and DTED-1) are public domain and may be obtained from
NASA via anonymous ftp at ftp://e0srp01u.ecs.nasa.gov/srtm/ or from the
USGS EROS Data Center via their Seamless Data Distribution System at
http://seamless.usgs.gov/.

2.5 SRTM Water Body Data (SWBD)

SRTM Water Body Data files are a by-product of the data editing
performed by NGA to produce the finished SRTM DTED-2 data. Ocean, lake
and river shorelines were identified and delineated from the 1
arc-second DTED-2 data (for details see NASA 2003) and were saved as
vectors in ESRI 3-D Shapefile format (ESRI 1998). There are
approximately 12,000 SWBD files since only those SRTM tiles that contain
water have a corresponding SWBD shapefile.

The guiding principle for the development of SWBD was that water must be
depicted as it was in February 2000 at the time of the Shuttle flight.
In most cases, two orthorectified SRTM image mosaics were used as the
primary source for water body editing. A landcover water layer and
medium-scale maps and charts were used as supplemental data sources.
Since the landcover water layer was derived mostly from Landsat 5 data
collected a decade earlier than the Shuttle mission and the map sources
had similar currency problems, there were significant seasonal and
temporal differences between the depiction of water in the SRTM data and
the ancillary sources. For more details see NASA/NGA (2003) and NASA
(2003).

2.6 Digital Chart of the World (DCW) global vectorized river network

The Digital Chart of the World (ESRI 1993) is a global vector map at a
resolution of 1:1 million that includes a layer of hydrographic features
such as rivers and lakes. DCW (also known as VMAP-0) is generally
considered to provide the most comprehensive and consistent global river
network data currently available. It is based on the US DMA (now NGA)
Operational Navigation Charts (ONC) whose information dates from the
1970s to the 1990s (Birkett and Mason 1995). The positional accuracy of
DCW varies considerably between regions, and there is no distinction
between natural rivers and artificial canals.

2.7 ArcWorld global vectorized river network

The ArcWorld data set (ESRI 1992) includes a global vector map of
surface water bodies at a resolution of 1:3 million. As part of its
classification scheme, it distinguishes linear rivers into natural
(perennial and intermittent) or artificial (canals) and provides
approximately 7000 polygons of large open water bodies (including rivers
and lakes). Although digitized at a coarser scale, ArcWorld seems to
include some corrections and updates as compared to DCW and provides a
consistent focus on major rivers and lakes of the world.

2.8 Global Lakes and Wetlands Database (GLWD)

The Global Lakes and Wetlands Database (Lehner and Döll 2004) combines a
variety of existing global lake and wetland maps (at 1:1 to 1:3 million
resolution) into one consistent coverage. It provides shoreline polygons
of approximately 250,000 lakes and reservoirs worldwide, including their
surface areas and other attributes. As for lakes and reservoirs, GLWD is
largely based on DCW and ArcWorld but also includes various updates and
data corrections.

**3. Data set development**

With all digital geospatial data sets, users must be aware of certain
characteristics of the data, such as resolution, accuracy, method of
production and any resulting artifacts, in order to better judge its
suitability for a specific application. A characteristic of the data
that renders it unsuitable for one application may have no relevance as
a limiting factor for its use in a different application (NASA/JPL
2005).

This section provides an overview of the applied processing steps for
the generation of HydroSHEDS and discusses some key technical
specifications in order to allow the user to better estimate the
suitability of the data set for a specific application. Additional data
validation details are addressed in section 4. Please also refer to the
flowchart of Appendix A.

3.1 Combination of unfinished SRTM-3 and finished DTED-1 data

*3.1.1 Combining SRTM-3 and DTED-1 original data*

For the generation of HydroSHEDS, the performance of the publicly
available SRTM-3 and DTED-1 versions of SRTM at 3 arc-second resolution
have been tested. Due to their specific characteristics, each data set
showed both advantages and disadvantages for hydrological applications.

As stated earlier, SRTM-3 has been derived through averaging of 1
arc-second SRTM data, as opposed to the subsampling method of DTED-1. As
averaging reduces the high frequency "noise" that is characteristic of
radar-derived elevation data, it is the method generally preferred by
the research community (NASA/JPL 2005).

On the other hand, SRTM-3 data does not represent open water surfaces
and shorelines well. DTED-1 has been specifically corrected to represent
these features. However, the correction protocol introduced some
critical artifacts for hydrological applications. For example, when
large rivers were identified and monotonically stepped down in height
towards the ocean, it was assured that the surface of each river pixel
was lower than that of the directly adjacent land pixels. Yet a slightly
elevated riverbank, say due to a levee or simply caused by the
interpretation of riparian vegetation in the radar image, may allow for
a river reach being somewhat higher than the floodplain behind the
riverbank. Since the original processing was performed at 1 arc-second
resolution, the elevated riverbank can disappear in the aggregated 3
arc-second version if it is only thin (one pixel wide). The resulting
effect in the derived flow direction map is a possible breakout of the
river course into the floodplain.

For above reasons, and after conducting a series of local tests, it was
decided to apply both SRTM-3 and DTED-1 data in combination. For each
pixel the minimum value found in either SRTM-3 or DTED-1 was used to
generate an initial HydroSHEDS elevation model. The minimum requirement
preserves the lower of both surfaces in the combined elevation data,
which is considered desirable for the later identification of drainage
directions.

*3.1.2 Ocean shoreline*

At the ocean surface, the combined data initially shows elevation values
of 0 (from DTED-1) or negative (from SRTM-3). Since land close to the
shoreline can also be 0 or even negative, using elevation alone as a
criterion does not allow for a clean identification of the ocean
shoreline. Thus to aid in the shoreline delineation, SWBD was employed
as ancillary data: where SWBD indicates "ocean", the values of the
HydroSHEDS elevation model were reclassified to no-data. The resulting
shoreline was then slightly generalized in order to remove small
artifacts: land was first extended by a one-pixel rim into the ocean and
the boundary was then smoothed using a local cell filter. All detached
ocean surfaces (e.g. small estuaries entirely surrounded by land cells)
were treated as land and their elevation values were retained rather
than set to no-data. Some larger rivers are defined in SWBD to extend
relatively far into the ocean. In these cases, the shoreline was
modified based on the shoreline of DCW. Some very small islands are
missing in the source data and are thus not represented in HydroSHEDS.
Finally, some minor errors were detected in SWBD in visual inspections
(e.g. some incomplete island boundaries) and were individually
corrected. Note that in all up-scaled HydroSHEDS layers, each cell that
contains at least one land cell at 3 arc-second resolution is defined as
land.

*3.1.3 Data shift*

Both SRTM-3 and DTED-1 original 1-degree by 1-degree data tiles are
defined via the coordinates of the center of their lower-left pixel (see
2.4). This characteristic leads to overlapping edges of adjacent tiles
and to some artifacts when aggregating a tile to coarser resolutions:
either all adjacent tiles have to be included in the aggregation
process, or overlapping edges may have to be eliminated in the result.
As the processing steps for the generation of HydroSHEDS are rather
complex and aggregation (scaling) plays an important role, it was
decided to shift the original SRTM data by 1.5 arc-seconds to the north
and east, and to remove each tile's overlapping right column and top
row. This shift leads to a 3 arc-second HydroSHEDS tile having 1200 rows
and 1200 columns at an extent of exactly 1-degree by 1-degree without
overlaps to adjacent tiles. All other HydroSHEDS resolutions are based
on the initial 3 arc-second data and, therefore, include this shift.
With respect to deriving river networks, the effect of the shift on the
accuracy of the data can be considered negligible, particularly when
compared to the subsequently applied data manipulations as discussed
below. Note, however, that the shift may lead to significant anomalies
when directly comparing HydroSHEDS elevation data and original SRTM
elevation data.

3.2 Void-filling

In its original release, SRTM data contains regions of no-data (voids),
specifically over large water bodies, such as lakes and rivers, and in
areas where radar-specific problems prevented the production of reliable
elevation data. These areas include mountainous regions where the radar
shadow effect is pronounced, such as the Himalayas and Andes, as well as
certain land surfaces, such as bare sand or rock conditions as found in
the Sahara Desert. The existence of no-data in the DEM causes
significant problems for deriving hydrological products, which require
continuous flow surfaces. Therefore, a void-filling procedure has been
applied to provide a continuous DEM for HydroSHEDS.

Numerous methods have been developed for void-filling of SRTM data (see
e.g., Gamache 2004a), but they rarely focus on specific hydrological
requirements. For HydroSHEDS, two different void-filling algorithms have
been applied in combination. The first has been developed by CIAT (see
Jarvis 2004; in collaboration with R. Hijmans and A. Nelson). The second
has been specifically developed for HydroSHEDS. While the CIAT algorithm
delivers smooth interpolation surfaces, the HydroSHEDS algorithm focuses
on low and flat-water surfaces. Both methods and their combination are
summarized below.

*3.2.1 CIAT algorithm*

The CIAT algorithm fills the no-data voids by applying an interpolative
technique. The original SRTM elevation data are used to produce contours
at an interval of 10 meters. The contours are interpolated using the
TOPOGRID algorithm in Arc/Info. TOPOGRID, based upon the established
algorithms of Hutchinson (1988, 1989), is designed to use contour and
point elevation data along with mapped hydrography to produce
hydrologically sound DEMs. This method produces a smooth elevation
surface within the no-data regions. While micro-scale topographic
variation is likely to be underrepresented, most macro-scale features
are captured well in small to intermediate sized voids. Jarvis et al.
(2004) performed a detailed analysis of the accuracy of the interpolated
elevation data for a region in Colombia and found little difference when
compared to a cartographic DEM, particularly for hydrological
applications. Gamache (2004b and personal communication) also analyzed
the CIAT results and concluded that the void-filling algorithm is quite
successful in representing broad scale patterns in topography. For the
production of Hydrosheds, Version 2 of the CIAT void-filled SRTM data
was used (CIAT 2004). The data is available from the CGIAR-CSI SRTM 90m
Database at http://srtm.csi.cgiar.org.

*3.2.2 HydroSHEDS algorithm*

The HydroSHEDS algorithm fills the no-data voids by means of an
iterative neighborhood analysis. The first step fills the outermost
pixel-rim of a no-data void using a combination of a 3x3 minimum and a
5x5 mean filter (the minimum filter dominates the mean filter by a
factor of 3:1). Then, the next pixel-rim is filled until the entire
no-data void is processed. The no-data area is finally smoothed using a
9x9 mean filter. Particularly in the case of lakes and large river
surfaces the emphasis of the minimum filter results in rather low
elevation values inside the voids and a relatively flat relief as small
peaks are successively filtered out.

*3.2.3 Combination of void-filling algorithms*

The lowering effect of the HydroSHEDS algorithm for open water surfaces
seems desirable for hydrological applications, as it tends to force the
flow course to stay within river channels and lakes. In mountainous
regions, however, the CIAT results are expected to better represent the
general topography. To optimize results, both algorithms were combined.
For each pixel the minimum value of either the CIAT or HydroSHEDS
algorithm was used. If, however, the HydroSHEDS algorithm computed
values more than 30 meters lower than CIAT, CIAT values minus 30 meters
were used.

In some large no-data voids entire mountains are lost using either of
the two filling methods. Therefore, starting at a distance of 0.03
degrees (approximately 3 km at the equator) from the rim of large voids,
elevation values were inserted from GTOPO30, a global DEM at 30
arc-second (approximately 1 km at the equator) resolution (Gesch et al.
1999). To avoid cliff effects, the inserted values were smoothed or
"feathered" in a 0.03-degree wide transition zone.

The filled voids were then merged into the initial HydroSHEDS elevation
data to provide a continuous elevation surface with no void regions. The
entire process was performed for each 1-degree by 1-degree tile with a
0.25-degree overlap to the eight adjacent tiles, thus ensuring seamless
transitions of topography even in areas with large voids.

The final result of steps 3.1 and 3.2 is

the HydroSHEDS void-filled elevation model (**DEM**) at 3 arc-second
resolution.

3.3 Sink identification

Typically, an original DEM will show a large number of sinks or
depressions. These are single or multiple pixels which are entirely
surrounded by higher elevation pixels. Some of these sinks are naturally
occurring on the landscape, representing endorheic (inland) basins with
no outlet to the ocean. In most cases, however, the sinks are considered
spurious, often caused by random and mostly small deviations in the
elevation surface. These anomalies occur even in high quality DEMs and
high resolutions due to DEM production methods. The spurious sinks are
critical problems in hydrological applications as they interrupt
continuous flow across the DEM surface. Therefore, sinks are typically
removed from the DEM before deriving a river network. Standard GIS
procedures have been developed to remove spurious sinks, and a common
approach is to raise the elevation values within the sinks until an
outflow point is encountered. Natural sinks can be forced to remain in
the DEM through "seeding", e.g. by putting a no-data cell into their
center.

As for HydroSHEDS, the definition of natural vs. spurious sinks has been
accomplished using a GIS-assisted manual process. All sinks of the
void-filled elevation model were identified in a standard GIS procedure,
and their maximum depth and extent were calculated. Sinks deeper than 10
meters and larger than 10 km^2^ were highlighted as "potential" natural
sinks. All regions of potential natural sinks were then inspected
visually and were either seeded or rejected. The decision was based on
information derived from DCW, ArcWorld, GLWD, and additional atlases and
maps. For example, a mapped "salt lake" with no obvious river draining
from it is considered a strong indication for an endorheic basin. The
visual inspections were performed at a zoom to 1-degree by 1-degree
windows, and several thousand naturally occurring sinks were identified
globally.

Obviously, the manual sink identification process is subjective, and in
many cases the definition of natural sinks is difficult and ambiguous.
Some depressions overflow periodically, following seasonal flooding
cycles, others spill only occasionally. Some large, relatively dry areas
may show numerous small depressions within a generally sloped surface,
and flow paths are poorly developed if at all (e.g. the Argentinean
Pampas and many desert areas). These depressions may or may not overflow
in a rain event. In some areas of no obvious drainage only some
"structural" sinks were placed at strategic locations. They do not
terminate the flow at all single depressions but at a final one to
indicate the endorheic character of the region. In karstic areas, rivers
may disappear in surface depressions, yet they can be closely connected
to a larger basin via underground pathways. In cases of large karstic
depressions, sinks were introduced, as it seemed easier for a user to
later remove the sinks and restore connectivity than to introduce them
from scratch. Artificial sinks, however, like large pits in surface
mining areas, were rejected.

3.4 Hydrologic conditioning

Besides sinks, original DEMs show a series of other characteristics,
artifacts and anomalies that can cause significant problems or errors in
hydrological applications. Some types of problems that are typical for
the SRTM elevation model are discussed in section 4. The most
significant characteristic is likely the fact that the elevation values
of SRTM, being a radar-derived product, are influenced by the vegetation
cover. In areas of low relief, these small deviations from the true
surface elevation can cause significant errors in the derived river
courses and flow directions.

In order to improve the performance of a DEM for hydrological
applications, a series of GIS processes and procedures exist and are
routinely applied. Yet due to the individual characteristics of
different DEMs and, on a global scale, due to the regional variations in
the type of errors, no one method exists that addresses all possible
problems. For HydroSHEDS, a sequence of hydrologic conditioning
procedures has been implemented, either adapted from standard GIS
functionality, newly developed, or customized. The general focus was to
strike a compromise between forcing the DEM to produce correct river
network topology, particularly for the largest of rivers, while
preserving as much original SRTM information as possible. Note that in
any case the conditioning process alters the original elevation data and
may render it unusable for other applications.

The following hydrologic conditioning procedures have been applied to
the HydroSHEDS elevation data:

*3.4.1 Deepening of open water surfaces*

All rivers and lakes as identified in SWBD were deepened by 10 meters in
order to force the derived flow to stay within these objects. As no-data
voids in the original SRTM elevation data may also indicate open water
surfaces (see 3.2), all void areas were lowered by 10 meters as well.
The 10-meter threshold was chosen as it imparts a strong enough effect
in flat areas (where the identification of river channels and lakes is
particularly difficult), while producing only insignificant changes in
areas with steeper slopes (where no-data voids are probably caused by
radar shadow rather than open water).

*3.4.2 Weeding of coastal zone*

In the radar-derived elevation model, mangrove or coastal vegetation
belts may be interpreted as a low but continuous embankment blocking any
direct outflow to the ocean. In the derived river network model, these
barriers can cause significant backwater effects. To reduce this effect,
the coastal zone, i.e. a 0.02-degree wide buffer (approximately 2 km at
the equator) along the ocean shoreline was "weeded" by reducing every
random third cell by 5 meters. This subtle change, in combination with
the following filters, forces occasional breakthroughs into the slightly
elevated coastal embankments.

*3.4.3 Stream burning*

The most extensive conditioning process in the generation of HydroSHEDS
has been the so-called "stream burning" procedure. Stream burning is an
often-used process to enforce known river courses into an elevation
surface. The elevation values along the rivers, as depicted e.g. in an
existing vector layer, are lowered by a certain value, thus "burning"
deep gorges into the elevation surface. The burning can be extended to
include a buffer around the river lines in order to shape a smoother
transition between the original surface and the gorge. For HydroSHEDS,
only large rivers and lakes were burned into the elevation surface in
order to avoid excessive alterations of the SRTM surface. All perennial
and intermittent rivers and lakes of ArcWorld, as well as all rivers and
lakes of GLWD were used, while the higher resolution but unclassified
DCW data was omitted. Since the accuracy of the existing global maps is
unknown, attempts were made to minimize the impact of these data sets on
the SRTM data. After multiple tests, the burning depth for rivers was
set to 12 meters, with a buffer of 0.005 degrees (approximately 500
meters at the equator) around the river courses. The burning depth was
reduced, in a stepwise manner, from 12 meters at the thalweg to 2 meters
at the edge of the buffer. Lakes were burned with a depth of 14 meters
and a buffer distance of 0.0025 degrees. The parameter setting aimed for
a noticeable forcing of the main rivers in flat areas, where otherwise
the correct delineation of rivers is difficult. In steep regions, the
small burning depth results in rather insignificant changes of the
elevation surface, hence the SRTM data remains the dominant information
for deriving drainage directions.

*3.4.4 Filtering*

The entire elevation surface was then filtered by applying a directional
3x3 neighborhood analysis. The elevation values of all possible straight
and obtuse angle flow paths in a 3x3 kernel were averaged and the
minimum value was assigned to the center cell. This filter aims to
remove remaining spikes and wells while preserving and enforcing linear
river courses and valley bottoms. In particular, single pixels that can
block a continuous flow path are removed.

*3.4.5 Molding of valley courses*

Next, valley courses were depicted through a neighborhood terrain
analysis and were deepened by 3 meters. The valleys were identified
through a 5x5 kernel median analysis combined with a grid-thinning
algorithm to detect linear features. This procedure of valley "molding"
has been specifically developed to improve river delineations in
tropical lowland areas by removing small obstacles in shallow valleys.
Due to the small deepening of 3 meters, no significant changes occur in
areas with stronger relief.

*3.4.6 Sink filling*

In a standard process, all spurious sinks in the elevation surface were
filled. Natural sinks were seeded in order to exclude them from removal
(see 3.3).

*3.4.7 Carving through barriers*

After sink filling, a river map was produced from the conditioned
elevation surface. All main river courses, defined as rivers with an
upstream catchment area of more than 1000 cells (approximately 8 km^2^
at the equator), were depicted. The main rivers were then projected onto
the initial HydroSHEDS elevation model, and all elevation rises along
the rivers when moving downstream were identified. These rising reaches
in the original elevation surface, which have obviously been removed
through filtering or sink-filling in the conditioning process, may
represent dams, bridges, embankments of any kind, or narrow gorges that
block the flow path. In many of these cases, the sink-filling effect
(i.e. the lifting and implicit flattening of the dammed area) may not be
desirable as any existing relief information within the filled area is
lost. To minimize this effect, a second conditioning iteration was
performed: first, all rising reaches along the main river courses were
leveled out in the initial elevation data by appropriately lowering
their respective heights, thus effectively "carving" through the
barriers. After this process, all other conditioning steps (3.4.1 to
3.4.6) were repeated.

During the entire conditioning process, hard- and software limitations
were reached due to the very large data size at 3 arc-second resolution.
All steps have therefore been performed on a tile-by-tile basis, with
extents between 1-degree by 1-degree and 5-degree by 5-degree. In order
to avoid edge effects, appropriate overlaps to the adjacent tiles were
added. In particular the sink-filling algorithm proved highly
susceptible to tile sizes and edge effects and had to be implemented in
an iterative approach. The processing was performed with an overlap of
up to 5 degrees (approximately 500 km at the equator) to adjacent tiles
to ensure seamless results without edge effects.

3.5 Manual corrections

The result of section 3.4 is a hydrologically conditioned elevation
surface at 3 arc-second resolution. From this elevation surface, a new
river network was derived and used for error checking. Because
computation of the river network at 3 arc-second resolution is very time
intensive, the data was first upscaled to 15 arc-second resolution
(approximately 500 meters at the equator; for upscaling see 3.6 below).
The derived river network was then compared visually to the rivers of
DCW, ArcWorld, and various atlases and paper maps.

Errors occurred particularly in flat areas with varying vegetation cover
(see section 4), such as floodplains and coastal zones. If the actual
rivers could be visually detected in the raw elevation data, their
courses were traced or adopted from the existing DCW river layer. These
rivers were then added to the stream layer used in the river burning
procedure of 3.4. In some areas, the given elevation values
significantly misrepresented the actual flow conditions (e.g. blocked
pathways due to narrow gorges, or inadequate filling of the no-data
voids of the original data). In these cases, the burning depth was
individually adjusted. Some other topological problems (e.g. diversions
into canals or multiple spillways of reservoirs) were treated in a
similar manner through introduction and adjustment of main pathways.
Actual flow channels of braided rivers and large river deltas could not
be topologically resolved due to the constraint of allowing only one
drainage direction per cell (the single flow direction algorithm does
not allow for river bifurcations). These zones have only been "cleaned"
to represent the main channel properly.

After detecting the errors and preparing the corresponding correction
data, all steps of 3.4 were repeated. In some areas, several iterations
of manual corrections were performed. As with the sink identification
process, the manual correction process is highly subjective. The visual
inspections were performed at a zoom to one-degree by one-degree
windows, and corrections were applied for several thousand locations
globally.

The final results of steps 3.4 and 3.5 are

\(1\) the HydroSHEDS hydrologically conditioned elevation model
(**CON**), and

\(2\) the HydroSHEDS drainage direction map (**DIR**) at 3 arc-second
resolution.

3.6 Upscaling

All procedures described in sections 3.1 to 3.5 were performed at 3
arc-second resolution. Yet for many applications, in particular
continental or global assessments, coarser resolutions are desirable as
they may significantly reduce calculation times while providing
acceptable accuracy. HydroSHEDS therefore delivers various resolutions,
from 3 arc-second to 5 minute. The coarser resolutions are all derived
from the 3 arc-second data through upscaling.

Upscaling drainage directions is not a straightforward process, as
typical aggregation methods, such as averaging of neighborhood kernels,
are not appropriate for directional values. A frequently applied
upscaling method is to first upscale the elevation data, and then derive
a new drainage direction map from this coarser DEM. This method is
generally fast and easy to perform, but it often delivers low-quality
results with respect to river network topology, due to the loss of
significant information in the aggregation process. An alternative
option is to first derive the river network at high resolution, and then
to upscale this network. This option preserves the network information,
which is most important for hydrological applications. However, it
requires complex procedures, which are difficult to realize at a global
scale and for the desired high resolutions. As a compromise, a combined
method has been developed and applied for generating HydroSHEDS. The
main steps in the upscaling process are as follows:

1.  The void-filled DEM is upscaled from the original 3 arc-second to
    the desired resolution. For this process, an algorithm was applied
    that calculates both the mean and minimum value found within the
    aggregation kernel and then takes the average. The minimum value is
    included in the calculation to emphasize valleys. Natural sinks were
    preserved in the upscaling process.

2.  A network of main rivers was calculated at 3 arc-second resolution.
    Main rivers were defined as those having an upstream catchment area
    of more than 1000 cells (approximately 8 km^2^ at the equator). The
    river network was derived for five-degree by five-degree tiles with
    a one-degree overlap to adjacent tiles to avoid edge effects.

3.  The main rivers were then burned into the upscaled elevation
    surface. The burning depth was defined as the sum of a constant (500
    meters) and a value dependent on the size of the respective river
    reach (0-400 meters, proportional to the logarithm of upstream
    cells). The relatively large burning depth assured that the river
    channels were preserved in the new elevation surface. No buffering
    was applied.

4.  Sinks were filled in the upscaled and burned elevation surface, and
    finally new drainage directions were calculated. Note that due to
    the strong burning, the elevation surface does not represent natural
    conditions any more. It is appropriate only for deriving drainage
    directions. To avoid confusion with true DEMs, the upscaled
    elevation surface is not offered as a standard HydroSHEDS product.

The upscaling process delivers a new drainage direction map (DIR) from
which a new river network can be derived. Due to the applied stream
burning, all main rivers (as defined in the upscaling process) should be
in very good alignment with the original river network. Only if two
close-by rivers drain through the same or adjacent upscaled cells, they
may be incorrectly merged into one flow channel. Smaller rivers, for
which no burning occurs, are based solely on the upscaled elevation
surface. Their quality may thus differ from the river network at 3
arc-second resolution.

The final results of step 3.6 are upscaled HydroSHEDS drainage direction
maps (**DIR**) at resolutions of 15 arc-second and 30 arc-second. Also,
a 5 minute product is in preparation.

3.7 Derived products

Ancillary HydroSHEDS products can be derived from the individual
drainage direction maps at their respective resolutions. These products
include flow accumulations, flow distances, river networks, and
watershed boundaries. A list of available HydroSHEDS data sets is
provided in section 5.

**4. Quality assessment**

At the present stage of developing HydroSHEDS, the final data quality
has not been evaluated systematically. Yet preliminary comparisons with
other global hydrographic data sets indicate the following:

-   Generally, HydroSHEDS shows significantly better accuracy than
    HYDRO1k, a global hydrographic data set at 1-km resolution (USGS
    2000), due to HydroSHEDS being based on a superior digital elevation
    model.

-   Generally, HydroSHEDS shows significantly better accuracy than the
    river layer of ArcWorld (even in difficult areas) as ArcWorld has
    been incorporated in the conditioning process of HydroSHEDS.

-   Generally, HydroSHEDS shows better accuracy than DCW. However, the
    accuracy of both data sets varies by location. In some regions where
    HydroSHEDS is particularly susceptible to errors, such as vegetated
    floodplains, the quality of DCW can be superior to HydroSHEDS.

-   Generally, HydroSHEDS does not reach the accuracy of high-resolution
    local river networks as depicted in existing maps or remote sensing
    imagery. The user is thus encouraged to further improve HydroSHEDS
    through incorporating local information.

Typically, river network products derived from digital elevation
surfaces are susceptible to various errors, foremost in flat regions
without well-defined relief. Additionally, the quality of HydroSHEDS
depends on the characteristics of the SRTM-based elevation model. Being
a radar product, SRTM elevation values are influenced by vegetation and
other surface effects, such as roughness, wetness, low backscatter
signal at open water surfaces and radar shadow (Freeman 1996). At this
point, known sources of errors in HydroSHEDS include:

-   Areas of low or not well-defined relief, including lake surfaces.

-   Varying vegetation cover, particularly in areas of low relief, e.g.
    large river floodplains. The radar signal is, at least partly,
    reflected from atop and within the vegetation cover and the returned
    signal is thus a complex mix of land surface elevation and
    vegetation height.

-   Low-relief coastal areas, in part due to the barrier effect of
    mangroves.

-   Large-scale roads or clearings in vegetation of low-relief areas.
    The lack of vegetation causes artificial depressions in the
    elevation surface.

-   Rivers less than 90 m wide enclosed by riparian vegetation. The
    vegetation effect can cause the river channel to appear slightly
    elevated.

-   Braided rivers and deltas. The use of the single flow direction
    algorithm does not allow for depiction of river bifurcations.

-   Narrow gorges. If the gorges are less than 90 m wide, they can
    appear closed on the elevation surface at 3 arc-second resolution.

-   Inland sinks and depressions. These are often ambiguous or temporary
    in nature. Additionally, in karst areas flow paths are not
    necessarily terminated at sinks due to possible underground
    connectivity, and artificial depressions like large-scale mining may
    have flow bypasses.

-   Elevated "barriers" in the elevation surface that in reality have no
    effect on flow continuity (e.g. bridges, high-density housing
    areas).

-   Areas of no-data voids in the original SRTM data. The larger the
    void, the more uncertain is the filled surface (see 3.2).

**5. Data layers and availability**

The HydroSHEDS data are being developed for all landmasses of the globe
north of 56° south (i.e. excluding Antarctica). The HydroSHEDS package
provides a suite of various raster and vector data sets, covering many
of the common derivative products used in hydrological analyses. The
data is prepared in seamless mode (no edge effects) and is provided as
global or continental maps, or in five-degree by five-degree tile
coverage.

HydroSHEDS is produced on a continental basis. The drainage direction
maps represent the core data layer from which users can derive their own
products by applying standard GIS functionality. All other data layers
are produced as time permits. Drainage direction maps are scheduled for
completion as follows:

  ------------------------------- ---------------------------------------
  South America                   complete (May 2006)

  Asia                            August 2006

  Europe                          October 2006

  Africa                          December 2006

  Australia                       March 2007

  North America                   June 2007
  ------------------------------- ---------------------------------------

At this time, HydroSHEDS provides selected core and auxiliary data sets
for South America at 3 arc-second and 15 arc-second resolutions,
including drainage directions and elevation surfaces. A vectorized
stream network is available at 15 arc-second resolution only. Future
data developments are scheduled to include basins; flow accumulations;
slope; aspect; wetness index; stream orders; flow distances up- and
downstream; as well as seamless global products at 30 arc-second and 5
minute resolution. Currently, the following data layers and resolutions
are available:

5.1 Void-filled digital elevation model

+------------------------+---------------------------------------------+
| Name signature         | **DEM**                                     |
+------------------------+---------------------------------------------+
| Data format            | Raster                                      |
+------------------------+---------------------------------------------+
| Values                 | Elevation in meters                         |
|                        |                                             |
|                        | (referenced to WGS84 EGM96 geoid)           |
+------------------------+---------------------------------------------+
| Projection             | Geographic (latitude/longitude)             |
|                        |                                             |
|                        | referenced to WGS84 horizontal datum        |
+------------------------+---------------------------------------------+
| Available resolutions  | 3 arc-second, 15 arc-second                 |
+------------------------+---------------------------------------------+

The elevation layers distributed with HydroSHEDS are based on a
combination of the original SRTM-3 and DTED-1 elevation models of SRTM
(for further specifications see 2.1 to 2.4 and 3.1). No-data voids have
been filled using interpolation algorithms, and the data has been
clipped at the ocean shoreline. Resolutions other than 3 arc-second are
derived through aggregation (averaging): each upscaled data pixel is
generated by averaging the corresponding neighborhood kernel of the 3
arc-second data. Note that for technical reasons HydroSHEDS elevation
data show a consistent shift of 1.5 arc-seconds to the north and east as
compared to original SRTM data (see 3.1.3).

SRTM elevation data is provided in geographic projection
(latitude/longitude) referenced to the WGS84 horizontal datum, and EGM96
vertical datum. Note that most global positioning systems use the WGS84
vertical datum as default. This difference in vertical datums may result
in absolute elevation differences between +106 m and -86 m. For precise
comparison of GPS elevation data with SRTM elevation data, conversion of
the vertical datum should be considered beforehand.

5.2 Hydrologically conditioned elevation

+------------------------+---------------------------------------------+
| Name signature         | **CON**                                     |
+------------------------+---------------------------------------------+
| Data format            | Raster                                      |
+------------------------+---------------------------------------------+
| Values                 | Elevation in meters                         |
|                        |                                             |
|                        | (referenced to WGS84 EGM96 geoid)           |
+------------------------+---------------------------------------------+
| Projection             | Geographic (latitude/longitude)             |
|                        |                                             |
|                        | referenced to WGS84 horizontal datum        |
+------------------------+---------------------------------------------+
| Available resolutions  | 3 arc-second                                |
+------------------------+---------------------------------------------+

The hydrologically conditioned elevation layers distributed with
HydroSHEDS are the result of an iterative conditioning and correction
process described in detail in section 3. For the specifics of the
underlying digital elevation model see 5.1. Note that the conditioning
process alters the original DEM and may render it incorrect for
applications other than deriving drainage directions. Endorheic basins
(inland sinks) are "seeded" with a no-data cell at their lowest point in
order to terminate the flow.

5.3 Drainage directions

  ------------------- ---------------------------------------------------
  Name signature      **DIR**

  Data format         Raster

  Values              Drainage directions in ESRI format (see below)

  Projection          Geographic (latitude/longitude) referenced to WGS84
                      datum

  Available           3 arc-second, 15 arc-second
  resolutions         
  ------------------- ---------------------------------------------------

The drainage direction maps distributed with HydroSHEDS define the
direction of flow from each cell in the conditioned DEM to its steepest
down-slope neighbor. Values of flow direction vary from 1 to 128. All
final outlet cells to the ocean are flagged with a value of 0. All cells
that mark the lowest point of an endorheic basin (inland sink) are
flagged with a value of -1. The flow direction values follow the
convention adopted by ESRI\'s flow direction implementation:

  ------------- --------------- ------------- --------------- -------------
  **32**                        **64**                        **128**

                                                           

  **16**                       **0**                        **1**

                                                           

  **8**                         **4**                         **2**
  ------------- --------------- ------------- --------------- -------------

5.4 Flow accumulation (number of cells)

  ------------------- ---------------------------------------------------
  Name signature      **ACC**

  Data format         Raster

  Values              Flow accumulation in number of cells (see below)

  Projection          Geographic (latitude/longitude) referenced to WGS84
                      datum

  Available           15 arc-second
  resolutions         
  ------------------- ---------------------------------------------------

The flow accumulation maps distributed with HydroSHEDS define the amount
of upstream area (in number of cells) draining into each cell. The
drainage direction layer is used to define which cells flow into the
target cell. The number of accumulated cells is essentially a measure of
the upstream catchment area. However, since the cell size of the
HydroSHEDS data set depends on latitude, the cell accumulation value
cannot directly be translated into drainage areas in square kilometers.
A flow accumulation map reflecting true catchment areas is in
preparation. Values range from 1 at topographic highs (river sources) to
very large numbers (on the order of millions of cells) at the mouths of
large rivers.

5.5 River network (stream lines)

  -------------- ---------------------- -----------------------------------------------
  Name signature **RIV**                

  Data format    Vector (lines)         

  Projection     Geographic             
                 (latitude/longitude)   
                 referenced to WGS84    
                 datum                  

  Resolutions    15 arc-second          

  Line           ID                     unique identifier
  attributes                            

                 Up_cells               max. flow accumulation (number of cells) of
                                        stream reach
  -------------- ---------------------- -----------------------------------------------

The river network layers distributed with HydroSHEDS are directly
derived from the drainage direction layers. The flow accumulation layer
is used for selection and attribution. Only rivers with upstream
drainage areas exceeding a certain threshold are selected: for the 15
arc-second resolution a threshold of 100 upstream cells has been used.
The vectorized river reaches are currently attributed with the maximum
flow accumulation (in number of cells) occurring within each river
reach. More attributes will be added in future versions.

5.6 Drainage basins (watershed boundaries)

  ---------------- ---------------------- -----------------------------------------
  Name signature   **BAS**                

  Data format      Vector (polygons)      

  Projection       Geographic             
                   (latitude/longitude)   
                   referenced to WGS84    
                   datum                  

  Resolutions      15 arc-second          

  Polygon          ID                     unique identifier
  attributes                              

                   Area_skm               surface area in square kilometers
  ---------------- ---------------------- -----------------------------------------

The drainage basin layers distributed with HydroSHEDS are not yet
finalized and will be provided at a later stage.

**6. Data formats and distribution**

6.1 File name syntax

HydroSHEDS provides data in various regional extents, types, and
resolutions. Information about the content is provided in the file names
which follow the naming convention "*Extent_DataType_Resolution*".

*6.1.1 Regional extent*

To facilitate electronic distribution, all raster data at 3 arc-second
resolution have been divided into five-degree by five-degree tiles. For
example, the area of South America is covered by 103 individual tiles.
The tile names are defined by a 7-digit identifier which refers to the
latitude and longitude of the lower-left (southwest) corner of the tile.
For example, the coordinates of the lower-left corner of tile s15w065
are 15 degrees south latitude and 65 degrees west longitude.

File names of layers with continental extent are defined by a two-digit
identifier:

  ------------------------ ----------------------------------------------
  *Identifier*             *Continent*

  Af                       Africa

  As                       Asia

  Au                       Australasia

  Eu                       Europe

  Na                       North America

  Sa                       South America
  ------------------------ ----------------------------------------------

*6.1.2 Data type*

  --------------- -------------------------------------------------------
  *Identifier*    *Type of data*

  DEM             Digital elevation model (void-filled)

  CON             Hydrologically conditioned elevation

  DIR             Drainage directions

  ACC             Flow accumulation (number of cells)

  RIV             River network (stream lines)

  BAS             Drainage basins (watershed boundaries)
  --------------- -------------------------------------------------------

*6.1.3 Resolution*

  -------------- -------------- -------------------- --------------------------
  *Identifier*   *in sec/min*   *in degree*          *in meters/km*

  3s             3 arc-second   0.0008333333333333   approx. 90 m at the
                                                     equator

  15s            15 arc-second  0.0041666666666667   approx. 500 m at the
                                                     equator

  30s            30 arc-second  0.0083333333333333   approx. 1 km at the
                                                     equator

  5m             5 minute       0.0833333333333333   approx. 10 km at the
                                                     equator
  -------------- -------------- -------------------- --------------------------

Please note that all data provided in five-degree by five-degree tiles
is in 3 arc-second resolution. However, the extension "3s" is omitted in
order to shorten the file names.

6.2 Data Formats

*6.2.1 Vector data format -- ESRI Shapefile format*

The vector data sets distributed with HydroSHEDS, i.e. river lines (RIV)
and basin polygons (BAS), are being made available in ESRI Shapefile
format (ESRI 1998). A HydroSHEDS shapefile typically consists of five
main files (.dbf, .sbn, .sbx, .shp, .shx). Additionally, basic metadata
information is provided both in XML format (.xml) and in HTML format
(.htm) following the FGDC standard. Projection information is provided
in an ASCII text file (.prj). All shapefiles are in geographic
(latitude/longitude) projection, referenced to datum WGS84.

*6.2.2 Raster data formats*

The raster data layers distributed with HydroSHEDS, i.e. the void-filled
elevation model (DEM), the hydrologically conditioned elevation model
(CON), drainage directions (DIR), and flow accumulation (ACC), are being
made available in two different formats: (a) in ESRI Grid format, and
(b) as binary raster image in ESRI Band Interleaved by Line (BIL)
format. The simple BIL format should allow for easy ingest into most
popular image processing and geographic information systems packages.
Further information on the contents of the raster files is provided
below.

*(a) ESRI Grid format*

The ESRI Grid format is supported by the ESRI GIS software environment,
but it can also be imported into various independent GIS packages. Each
raster coverage is provided in a root folder (named after the raster
layer) which contains two subfolders: the grid data folder (again named
after the raster layer) and a corresponding "info" folder. In the grid
folder, basic metadata information is provided both in XML format (.xml)
and in HTML format (.htm) following the FGDC standard. Projection
information is provided in an ASCII text file (prj.adf). All raster data
are in geographic (latitude/longitude) projection, referenced to datum
WGS84.

Note that it is not possible to store two or more grids in the same root
folder by simply copying the according grid folders and merging the
content of the "info" folders into one. To store multiple grids in one
root folder, the grids have to be copied or moved from within an ESRI
compatible software application.

*(b) ESRI BIL format*

Each raster image is provided as four main files, with the extension of
each file defining the file type (see table below). Additionally, basic
metadata information is provided both in XML format (.xml) and in HTML
format (.htm) following the FGDC standard. Projection information is
provided in an ASCII text file (.prj). All raster data are in geographic
(latitude/longitude) projection, referenced to datum WGS84.

  ------------------------------ ----------------------------------------
  *Extension*                    *File type*

  .bil                           Raster data file

  .hdr                           Header file

  .blw                           World file

  .stx                           Statistics file
  ------------------------------ ----------------------------------------

[Raster data file (.bil)]{.underline}

The raster data for each layer are provided as unsigned integer data in
a simple binary raster format (either 8-bit, 16-bit, or 32-bit). There
are no header or trailer bytes embedded in the image. The data are
stored in row major order (all the data for row 1, followed by all the
data for row 2, etc.).

[Header file (.hdr)]{.underline}

The raster data header file is an ASCII text file containing size and
coordinate information for the layer. Many standard software packages
require the header file to provide important geo-referencing information
for the image. The following keywords are used in the header file:

+------------------+---------------------------------------------------+
| BYTEORDER        | byte order in which image pixel values are stored |
|                  |                                                   |
|                  | I = Intel byte order (least significant byte      |
|                  | first)                                            |
+------------------+---------------------------------------------------+
| LAYOUT           | organization of the bands in the file             |
|                  |                                                   |
|                  | BIL = band interleaved by line                    |
+------------------+---------------------------------------------------+
| NROWS            | number of rows in the image                       |
+------------------+---------------------------------------------------+
| NCOLS            | number of columns in the image                    |
+------------------+---------------------------------------------------+
| NBANDS           | number of spectral bands in the image (1 for all  |
|                  | raster layers)                                    |
+------------------+---------------------------------------------------+
| NBITS            | number of bits per pixel (8, 16 or 32)            |
+------------------+---------------------------------------------------+
| BANDROWBYTES     | number of bytes per band per row (twice the       |
|                  | number of columns for the 16-bit image;           |
|                  | four-times for the 32-bit image)                  |
+------------------+---------------------------------------------------+
| TOTALROWBYTES    | total number of bytes of data per row (twice the  |
|                  | number of columns for the 16-bit image;           |
|                  | four-times for the 32-bit image)                  |
+------------------+---------------------------------------------------+
| BANDGAPBYTES     | the number of bytes between bands in a BSQ format |
|                  | image (0 for all raster layers)                   |
+------------------+---------------------------------------------------+
| NODATA           | Value used for masking purposes (-9999 for all    |
|                  | elevation images DEM and CON; -9 for all drainage |
|                  | direction maps DIR; 0 for all flow accumulation   |
|                  | maps ACC)                                         |
+------------------+---------------------------------------------------+
| ULXMAP           | longitude of the center of the upper-left pixel   |
|                  | (decimal degrees)                                 |
+------------------+---------------------------------------------------+
| ULYMAP           | latitude of the center of the upper-left pixel    |
|                  | (decimal degrees)                                 |
+------------------+---------------------------------------------------+
| XDIM             | x-dimension of a pixel in geographic units        |
|                  | (decimal degrees)                                 |
+------------------+---------------------------------------------------+
| YDIM             | y-dimension of a pixel in geographic units        |
|                  | (decimal degrees)                                 |
+------------------+---------------------------------------------------+

[World file (.blw)]{.underline}

The world file is an ASCII text file containing coordinate information.
It is used by some packages for geo-referencing of image data. The
following parameters are provided in the world file:

  -------------- --------------------------------------------------------
  XDIM           x-dimension of a pixel in geographic units (decimal
                 degrees)

  Rotation term  always zero

  Rotation term  always zero

  Negative YDIM  negative y-dimension of a pixel in geographic units
                 (decimal degrees)

  XMIN           longitude of the center of the upper-left pixel (decimal
                 degrees)

  YMAX           latitude of the center of the upper-left pixel (decimal
                 degrees)
  -------------- --------------------------------------------------------

[Statistics file (.stx)]{.underline}

The statistics file is an ASCII text file which lists the band number,
minimum value, maximum value, mean value, and standard deviation of the
values in the raster data file.

[Important note for using BIL files:]{.underline}

Because the BIL image data (.bil) are stored in binary format, users
must be aware of how the bytes are addressed on their computers. The
data are provided in Intel byte order, which stores the least
significant byte first ("little endeian"). Most PCs use the Intel byte
order. However, systems such as Sun SPARC and Silicon Graphics
workstations use the Motorola byte order, which stores the most
significant byte first (\"big endian\"). Users with systems that address
bytes in the Motorola byte order may have to \"swap bytes\" of the BIL
data unless their application software performs the conversion during
ingest. The statistics file (.stx) provided for each data set gives the
range of values in the image file, so that users can check if they have
the correct values stored on their system.

Because the BIL images are provided in unsigned binary format, negative
values are stored in reverse order starting at the top end of the
possible value range for each data file. For example, HydroSHEDS
drainage direction maps (DIR) are stored as unsigned 8-bit data,
allowing for values between 0 and 255, and the value -1 is stored as
255, -2 is 254, etc. HydroSHEDS elevation layers (DEM and CON) are
stored as unsigned 16-bit data, allowing for values between 0 and 65535.
Also, ocean areas have been masked in HydroSHEDS, using NoData flags of
-9999 (DEM and CON layers) and -9 (DIR layer). Many software
applications cannot directly interpret these negative values. An easy
fix can be accomplished by converting the original BIL image using a
formula like the following (from ArcInfo):

For 8-bit data: out_grid = con(in_grid \>= 128, in_grid - 256, in_grid)

For 16-bit data: out_grid = con(in_grid \>= 32768, in_grid - 65536,
in_grid)

The converted grid will then have the negative values properly
represented, and the statistics of the grid should match those listed in
the .stx file. If desired, the -9999 or -9 ocean mask values in the grid
can then be set to NoData. The HydroSHEDS flow accumulation maps (ACC),
stored as unsigned 32-bit data, do not include negative values and the
value 0 masks ocean areas.

6.3 Data distribution

HydroSHEDS data is available electronically in compressed zip file
format from the EROS Data Center at
[http://hydrosheds.cr.usgs.gov](http://hydrosheds.cr.usgs.gov/) (at no
cost). To use the data files, the zip files must first be decompressed.
Each zip file includes a copy of the HydroSHEDS Technical Documentation.
Multiple zip files that contain data in ESRI Shapefile or BIL format can
be decompressed into one folder. Multiple zip files that contain data in
ESRI Grid format are by default decompressed into separate subfolders.
To store multiple grids in one root folder, the grids have to be copied
or moved from within an ESRI compatible software application.

Below are some estimates for file sizes of the compressed zip files.
Decompressed, the various file sizes may exceed the zipped sizes by
10-fold and more.

  ------------------------------------------- ---------------------------
  *Zipped file*                               *Estimated size range*

  Five-degree by five degree tiles            10-50 MB

  Continental grids at 15s resolution         30-100 MB

  Continental grids at 30s resolution         10-50 MB

  Continental vector files                    10-50 MB
  ------------------------------------------- ---------------------------

**7. Notes for HydroSHEDS users**

This section provides notes and hints for users of HydroSHEDS and will
be updated periodically.

FAQs:

**Q 01:** I have additional local information available in form of
high-quality river maps. How can I incorporate this information in
HydroSHEDS?

**A 01:** First, compare your local maps with the HydroSHEDS river
network. If there are only a few differences, you can depict the
respective river lines from the local map and burn them into the
*hydrologically conditioned* DEM of HydroSHEDS. Then run a sink-fill
process and derive a new drainage direction map and river network from
the corrected and conditioned DEM. If the local river map shows
generally better quality at many locations, or if the conditioning
process of HydroSHEDS introduced an error that is not apparent in the
raw SRTM data, you can use the local river map and burn it into the
*void-filled* (un-conditioned) DEM of HydroSHEDS. However, you will then
have to apply your own individual conditioning processes (at a minimum
you will have to apply a sink-fill procedure) in order to produce a new
hydrologically conditioned DEM.

**Q 02:** How can I remove an incorrect sink from HydroSHEDS and restore
continuous flow?

**A 02:** Use the *hydrologically conditioned* DEM of HydroSHEDS.
Reclassify the no-data cell that represents the incorrect sink with an
appropriate elevation value. \[In ArcView 3.x you can achieve this as
follows: make a new point theme; place a point at the location of the
no-data cell and enter the desired elevation value into the attribute
table; convert the point theme to a grid and merge it into the DEM.\]
Then run a sink-fill process and derive a new drainage direction map and
river network from the corrected and conditioned DEM.

**8. References**

Birkett, C.M., Mason, I.M. (1995): A new global lakes database for a
remote sensing program studying climatically sensitive large lakes.
Journal of Great Lakes Research 21(3): 307-318.

CIAT -- International Centre for Tropical Agriculture (2004):
Void-filled seamless SRTM data V2. Data available from the CGIAR-CSI
SRTM 90m Database at http://srtm.csi.cgiar.org

ESRI -- Environmental Systems Research Institute (1992): ArcWorld 1:3
Mio. Continental Coverage. ESRI, Redlands, CA. Data available on CD.

ESRI -- Environmental Systems Research Institute (1993): Digital Chart
of the World 1:1 Mio. ESRI, Redlands, CA. Data available on 4 CDs or at
http://www.maproom.psu.edu/dcw/

ESRI -- Environmental Systems Research Institute (1998): ESRI Shapefile
Technical Description - An ESRI white paper. Available at
http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf

Farr, T.G., M. Kobrick (2000): Shuttle Radar Topography Mission produces
a wealth of data. American Geophysical Union, Eos 81: 583-585.

Freeman, T. (1996): What is Imaging Radar? Jet Propulsion Laboratory.
Document available at
http://southport.jpl.nasa.gov/desc/imagingradarv3.html

Gamache, M. (2004a): Free and low cost datasets for international
mountain cartography. Documentation for the Alpine Mapping Guild, 42 pp.
Available at http://www.icc.es/workshop/abstracts/ica_paper_web3.pdf

Gamache, M. (2004b): Various documents including evaluations of CIAT's
void-filling methodology, available at:
http://www.icc.es/workshop/presentacions/s5/1_martin/presentation.pdf
and http://www.alpinemapguild.com/ICA_CMC2004/Gamache final web.pdf

Gesch, D.B., Verdin, K.L., Greenlee, S.K. (1999): New land surface
digital elevation model covers the Earth. Eos. Transactions, American
Geophysical Union 80(6): 69-70.

Hutchinson, M. (1988): Calculation of hydrologically sound digital
elevation models. Third International Symposium on Spatial Data
Handling, Columbus, Ohio, International Geographical Union.

Hutchinson, M. (1989): A new procedure for gridding elevation and stream
line data with automatic removal of spurious pits. Journal of Hydrology
106: 211-232.

Jarvis, A., J. Rubiano, A. Nelson, A. Farrow, Mulligan M. (2004):
Practical use of SRTM data in the tropics: comparisons with digital
elevation models generated from cartographic data. Working Document no.
198. Cali, International Centre for Tropical Agriculture (CIAT): 32 pp.

Jarvis, A. (2004): SRTM Data Processing Methodology. Available at
http://srtm.csi.cgiar.org/SRTMdataProcessingMethodology.asp

Lehner, B., Döll, P. (2004): Development and validation of a global
database of lakes, reservoirs and wetlands. Journal of Hydrology
296(1-4): 1-22. Data available at
http://www.wwfus.org/science/data/globallakes.cfm

NASA (2003): SRTM Data Editing Rules, Version 2.0. 23 pp. Available at
ftp://e0srp01u.ecs.nasa.gov/srtm/version2/Documentation/SRTM_edit_rules.pdf
or at http://edc.usgs.gov/products/elevation/swbdedit.doc

NASA/JPL (2005): SRTM Topography (SRTM documentation). 8pp. Available at
ftp://e0srp01u.ecs.nasa.gov/srtm/version2/Documentation/SRTM_Topo.pdf

NASA/NGA (2003): SRTM Water Body Data Product Specific Guidance, Version
2.0. 4pp. Available at
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USGS -- U.S. Geological Survey (2000): HYDRO1k Elevation Derivative
Database. USGS EROS Data Center, Sioux Falls, SD. Available at
http://edc.usgs.gov/products/elevation/gtopo30/hydro/

**9. Disclaimer**

Copyright and redistribution restrictions

HydroSHEDS has been developed by the Conservation Science Program of
World Wildlife Fund (WWF), hereafter referred to as "authors", in
partnership with the U.S. Geological Survey (USGS), the International
Centre for Tropical Agriculture (CIAT), The Nature Conservancy (TNC),
and the Center for Environmental Systems Research (CESR) of the
University of Kassel, Germany.

Users may apply HydroSHEDS for non-commercial purposes. Any modification
of the original data by users must be noted. The authors of HydroSHEDS
may request reprints of publications and copies of derived materials.
The user shall not reproduce, convert, (re)publish, (re)distribute,
(re)broadcast, (re)transmit, sell, rent, lease, sublicense, lend,
assign, time-share, or transfer, in whole or in part, or provide
unlicensed third parties access to the data and related materials
without explicit written permission from the authors.

HydroSHEDS has been generated through the extensive use and
incorporation of data products provided by various other authors (see
"Data sources" and "Data set development"). The data and related
materials may thus contain proprietary and confidential property of
these authors and/or their licensor(s). HydroSHEDS data and related
materials are protected by United States copyright or sui generis laws
and applicable international copyright treaties and/or conventions.

Disclaimer of Warranty

The data and any related materials contained therein are provided "as
is," without warranty of any kind, either express or implied, including,
but not limited to, the implied warranties of merchantability, fitness
for a particular purpose, noninterference, system integration, or
noninfringement. The entire risk of use of the data shall be with the
user. The user expressly acknowledges that the data may contain some
nonconformities, defects, or errors. The authors do not warrant that the
data will meet the user\'s needs or expectations, that the use of the
data will be uninterrupted, or that all nonconformities, defects, or
errors can or will be corrected. The authors are not inviting reliance
on these data, and the user should always verify actual data.

Limitation of Liability

In no event shall the authors be liable for costs of procurement of
substitute goods or services, lost profits, lost sales or business
expenditures, investments, or commitments in connection with any
business, loss of any goodwill, or for any direct, indirect, special,
incidental, exemplary, or consequential damages arising out of the use
of the data and any related materials, however caused, on any theory of
liability, and whether or not the authors have been advised of the
possibility of such damage. These limitations shall apply
notwithstanding any failure of essential purpose of any exclusive
remedy.

Disclaimer by USGS

Any use of trade, product, or firm names is for descriptive purposes
only and does not imply endorsement by the U.S. Government. Please note
that some information contained in this data set and documentation may
be preliminary in nature and presented prior to final review and
approval by the Director of the USGS.

ACKNOWLEDGMENT AND CITATION

We kindly ask users to cite HydroSHEDS in any published material
produced using this data. If possible, please provide online links to
<http://www.worldwildlife.org/hydrosheds> for general information,
and/or [http://hydrosheds.cr.usgs.gov](http://hydrosheds.cr.usgs.gov/)
for data download and technical information.

Citations should be made as follows:

Lehner, B., Verdin, K., Jarvis, A. (2006): HydroSHEDS Technical
Documentation. World Wildlife Fund US, Washington, DC. Available at
[http://hydrosheds.cr.usgs.gov](http://hydrosheds.cr.usgs.gov/).

**Appendix A: Flowchart of the generation of HydroSHEDS**

(for abbreviations and further explanations see text)