U.S. Department of the Interior

U.S. Geological Survey

Bathymetry and Digital Elevation Models of Coyote Creek 

and Alviso Slough, South San Francisco Bay, California

By Amy C. Foxgrover, David P. Finlayson, Bruce E. Jaffe, and Theresa A. Fregoso

Open-File Report 2011–1315

Version 4.0, March 2018 
ii

U.S. Department of the Interior

RYAN ZINKE, Secretary

U.S. Geological Survey

William H. Werkheiser, Deputy Director

 exercising the authority of the Director

U.S. Geological Survey, Reston, Virginia: 2011

Revised: April 2012, March 2014, September 2015, March 2018

For more information on the USGS—the Federal source for science about the Earth,

its natural and living resources, natural hazards, and the environment—visit

https://www.usgs.gov/ or call 1–888–ASK–USGS (1–888–275–8747).

For an overview of USGS information products, including maps, imagery, and publications,

visit https://store.usgs.gov/.

Any use of trade, firm, or product names is for descriptive purposes only and does not imply

endorsement by the U.S. Government.

Although this information product, for the most part, is in the public domain, it also may

contain copyrighted materials as noted in the text. Permission to reproduce copyrighted 

items

must be secured from the copyright owner.

Suggested citation: 

Foxgrover, A.C., Finlayson, D.P., Jaffe, B.E., and Fregoso, T.A., 2018, Bathymetry and Digital Elevation Models of 

Coyote Creek and Alviso Slough, South San Francisco Bay, California (ver. 4.0, March, 2018): U.S. Geological 

Survey Open-File Report 2011-1315, 24 p., https://doi.org/10.3133/ofr20111315. 

SSN 2331-1258 (online)

Cover: Seamless bathymetric/topographic DEM of Coyote Creek and Alviso Slough, south San Francisco Bay,

California; from figure 9. 
iii

Contents

Abstract ......................................................................................................................................................... 1

Data Collection .............................................................................................................................................. 1

Geodetic Control ........................................................................................................................................ 4

Sound Velocity Measurements................................................................................................................... 5

Processing Procedures.................................................................................................................................. 5

Real-Time Sonar Sounding Processing ..................................................................................................... 6

Backscatter Image Production.................................................................................................................... 7

Swath Cleaning and Filtering...................................................................................................................... 7

Digital Elevation Model Production ................................................................................................................ 8

Bathymetric Grids....................................................................................................................................... 8

2010 Lidar Data.......................................................................................................................................... 9

2010 Bathymetric / Topographic DEM...................................................................................................... 10

Horizontal and Vertical Datum Conversions............................................................................................. 13

Estimates of Bathymetric Uncertainty .......................................................................................................... 14

Vertical Adjustments to Post-2010 Surveys.............................................................................................. 16

Data Tables ................................................................................................................................................. 17

Bathymetry ............................................................................................................................................... 17

2010 Bathymetric / Topographic DEM...................................................................................................... 18

A Note on Coordinate Systems and Datums............................................................................................ 19

Acknowledgments........................................................................................................................................ 19

References Cited......................................................................................................................................... 19

Appendix A .................................................................................................................................................. 20

Appendix B .................................................................................................................................................. 24

Figures

Figure 1. Map of study area and location of GPS base station, south San Francisco Bay, California. ................... 2

Figure 2. The U.S. Geological Survey, Pacific Coastal and Marine Science Center R/V Parke Snavely. Photo 

courtesy of Thomas E. Reiss......................................................................................................................................... 3

Figure 3. Fore and aft views of the SWATHplus sonar pole mount on the U.S. Geological Survey Pacific 

Coastal and Marine Science Center R/V Parke Snavely. .............................................................................................. 3

Figure 4. Processing workflow diagram for converting soundings to a digital elevation model............................... 6

Figure 5. Map of backscatter amplitude (raw 16-bit digital number, DN), south San Francisco Bay, California...... 7

Figure 6. The 1 m resolution 2010 bathymetry grid generated by merging surveys S-2-10-SF, S-18-10-SF and 

S-24-10-SF. Elevation in meters relative to NAVD88. ................................................................................................... 8
iv

Figure 7. Bathymetric change from 2010 to October 2016. .................................................................................... 9

Figure 8. Difference between tidal flat elevations calculated from 2010 aerial lidar data versus bathymetry data, 

south San Francisco Bay, California............................................................................................................................ 11

Figure 9. The 2010 seamless bathymetric/topographic DEM of the region surrounding Coyote Creek and 

Alviso Slough, south San Francisco Bay, California. Elevations relative to NAVD88................................................... 12

Figure 10. The 2010 seamless bathymetric/topographic DEM of Coyote Creek and Alviso Slough clipped to the 

approximate extent of the shoreline, south San Francisco Bay, California. Elevations relative to MLLW.................... 12

Figure 11. Plan-view map of bathymetric soundings standard deviation within each 1 × 1 m cell, south San 

Francisco Bay, California............................................................................................................................................. 15

Figure 12. Histogram of sounding elevation standard deviations within each 1 × 1 m cell. .................................... 15

Tables

Table 1. Bathymetric cruise IDs and survey dates................................................................................................. 2

Table 2. SWATHplus-M sonar specifications (Systems Engineering and Assessment, Ltd., 2004). ..................... 4

Table 3. Global positioning system base-station benchmark................................................................................. 4

Table 4. Parameters adopted for transformation between NAD83(CORS96) and ITRF2000.............................. 13

Table 5. Cruise dates, type of Inertial Measurement Unit (IMU) used, number of SWATHplus transducers 

collecting data, and net vertical adjustment applied to align surveys with the same vertical reference plane as the 

2010 baseline survey................................................................................................................................................... 17

Table 6. Bathymetric data files provided.............................................................................................................. 17

Table 7. Bathymetric/topographic DEM files provided......................................................................................... 18
Bathymetry and Digital Elevation Models of Coyote Creek 

and Alviso Slough, South San Francisco Bay, California

By Amy C. Foxgrover, David P. Finlayson, Bruce E. Jaffe and Theresa A. Fregoso

Abstract

In 2010 the U.S. Geological Survey (USGS), Pacific Coastal and Marine Science Center

completed three cruises to map the bathymetry of the main channel and shallow intertidal mudflats in 

the southernmost part of south San Francisco Bay. The three surveys were merged to generate

comprehensive maps of Coyote Creek (from Calaveras Point east to the railroad bridge) and Alviso 

Slough (from the bay to the town of Alviso) to establish baseline bathymetry prior to the breaching of 

levees adjacent to Alviso and Guadalupe Sloughs as part of the South Bay Salt Pond Restoration Project

(http://www.southbayrestoration.org). Since 2010 the USGS has conducted twelve additional surveys 

to monitor bathymetric change in this region as restoration progresses. 

The bathymetry surveys were conducted using the state-of-the-art research vessel R/V Parke 

Snavely outfitted with an interferometric sidescan sonar for swath mapping in extremely shallow 

water. This publication provides high-resolution bathymetric data collected by the USGS. For the 2010 

baseline survey we have merged the bathymetry with aerial lidar data that were collected for the

USGS during the same time period to create a seamless, high-resolution digital elevation model (DEM) 

of the study area. The series of bathymetry datasets are provided at 1 m resolution and the 2010 

bathymetric/topographic DEM at 2 m resolution. The data are formatted as both X, Y, Z text files and 

ESRI Arc ASCII files that are accompanied by Federal Geographic Data Committee (FGDC) compliant 

metadata.

Data Collection

Fifteen high-resolution bathymetric surveys were collected in the southernmost reaches of 

south San Francisco Bay between January 2010 and March 2017 (table 1). The three surveys collected 

in 2010 were combined into a single composite surface of pre-breach baseline bathymetry covering 

the main channel, shallow intertidal mudflats, and Alviso and Guadalupe Sloughs. Since 2010 twelve

additional surveys have been performed to monitor bathymetric change in this region as restoration 

progresses. The surveys extend east from Calaveras Point along Coyote Creek to the railroad bridge, 

along Alviso Slough to the town of Alviso (slightly more than 7 km), and along the 3.7 km of Guadalupe 

Slough closest to the bay (fig. 1). The spatial coverage of the surrounding intertidal flats varies by 

survey, as accessibility by boat is highly dependent upon the tides. 
2

Table 1. Bathymetric cruise IDs and survey dates.

USGS Field Activity ID Cruise Dates

S-2-10-SF1 1/13/2010 – 1/15/2010

S-18-10-SF1 9/11/2010 – 9/13/2010

S-24-10-SF1 12/3/2010

S-10-11-SF 10/24/2011 – 10/30/2011

S-02-12-SF 2/3/2012 – 2/9/2012

S-03-12-SF 4/2/2012 – 4/6/2012

S-06-12-SF 10/12/2012 – 10/19/2012

S-05-13-SF 4/2/2013 – 4/3/2013, 4/23/2013 – 4/25/2013

S-08-13-SF 11/1/2013 – 11/7/2013

2014-670-FA 10/23/2014 – 10/24/2014

2015-633-FA 4/26/2015 – 4/27/2015

2015-669-FA 10/13/2015 – 10/16/2015

2016-628-FA 4/5/2016 – 4/6/2016

2016-678-FA 10/12/2016 – 10/13/2016, 10/15/2016 – 10/19/2016

2017-628-FA 3/28/2017 – 3/29/2017

1

Surveys combined into a single 2010 bathymetric surface.

Figure 1. Map of study area and location of GPS base station, south San Francisco Bay, California.

All of the surveys were done aboard the R/V Parke Snavely outfitted with a 234.5 kHz SEA 

(Systems Engineering & Assessment, Ltd.) SWATHplus-M phase-differencing sidescan sonar (figs. 2 and 

3). Global positioning system (GPS) data were passed through either a CodaOctopus F180 inertial 

measurement unit (IMU) or an Applanix Position and Orientation System for Marine Vessels (POS MV) 

to the sonar hardware and data collection software. Sonar heads, GPS antennae, and the IMU were 

surveyed in place to a common reference frame with a Geodimeter 640 Total Station. The R/V Parke 

Snavely was outfitted with three networked workstations and a navigation computer for use by the 

captain and survey crew for data collection and initial processing. See table 2 for the sonar system 

specifications.
3

Figure 2. The U.S. Geological Survey, Pacific Coastal and Marine Science Center R/V Parke Snavely. Photo 

courtesy of Thomas E. Reiss.

Figure 3. Fore and aft views of the SWATHplus sonar pole mount on the U.S. Geological Survey Pacific Coastal 

and Marine Science Center R/V Parke Snavely.
4

Table 2. SWATHplus-M sonar specifications (Systems Engineering and Assessment, Ltd., 2004).

Sonar frequency 234.5 kHz

Maximum water depth 120 m

Maximum swath width 300 m (typically 7 to 12 times water depth)

Resolution across track (best case) 5 cm

Transmit pulse length 34 to 500 ms

Ping repetition rate

150 m swath width 10 pings per second

300 m swath width 5 pings per second

Vertical accuracy (range dependent)

57 m 0.1 m

114 m 0.2 m

171 m 0.3 m

Geodetic Control

Geodetic control for the 2010-2012 surveys was established using a shore based GPS base 

station broadcasting real-time kinematic (RTK) corrections to the survey vessel by UHF radio link. The 

base station was at Moffett Naval Air Station, on a pre-existing benchmark identified as ARC 34 (fig. 1; 

table 3). The National Geodetic Survey (NGS) lists this monument as PID DG6881 (see appendix A for 

NGS datasheet). 

Table 3. Global positioning system base-station benchmark.

Reference frame NAD83 (NSRS2007)

Latitude N 37∘ 25’ 34.57880" 

Longitude W 122∘ 02’ 05.53373"

Orthometric height 1.28 m (NAVD88 height modernization project elevation) 

Epoch date 2007.00 

Prior to 2013 the R/V Parke Snavely was equipped with a CodaOctopus F180 attitude and 

positioning system. The F180 runs F190 firmware, and receives RTK positioning corrections directly.

The RTK GPS data (2 cm error ellipse) are combined with the inertial motion measurements directly 

within the F190 hardware so that high-precision position and attitude corrections are fed in real time

to the sonar acquisition equipment. The NAD83 (NSRS2007) Epoch 2007.00 3-dimensional reference 

frame was used for horizontal positioning, with elevations referenced to NAVD88. All data are

projected in UTM coordinate space in meters, zone 10 north.

In 2013, the F180 IMU was replaced by an Applanix Position and Orientation System for Marine 

Vessels (POS MV) which eliminated the need to set up an RTK base station. The POS MV utilizes global 

navigation satellite system (GNSS) data in combination with angular rate and acceleration data from 

the IMU, and heading data from the GPS Azimuth Measurement Systems (GAMS) to produce accurate 

position and orientation information through a virtual network of base stations. As opposed to 
5

receiving high-accuracy RTK corrections in real time, the POS records raw inertial and GNSS data while 

surveying that is later refined through post processing to incorporate publicly available GPS data from 

nearby base stations. During post processing the POS MV data is run through POSPac software to 

produce a smoothed best estimate of trajectory (SBET) file, which is then imported back into Swath

Processor to produce high-accuracy positions relative to the WGS84 ellipsoid. The root mean square 

error (RMSE) results from our POS MV surveys show positional errors of less than 5 cm in the X, Y, and 

Z directions.

We conducted a series of tests to assess any potential biases that may have been introduced by 

switching from the F180 to POS MV. In April and November of 2013 we conducted repeat bathymetric 

surveys of intertidal flats using both systems. On April 23, 2013, we surveyed 66,000 m2 of the Alviso 

intertidal flats with the POS MV and we returned to the same location two days later and surveyed

with the F180. On November 1, 2013, we surveyed nearly 170,000 m2 of tidal flats just south of the 

Dumbarton Bridge, which is approximately 8 km northwest of the Alviso study site, with the F180. The 

following day we surveyed the same location using the POS MV. All survey instrumentation and 

settings were identical with the exception of the IMUs. To assess differences between the repeat 

surveys, the bathymetric tracklines were aggressively trimmed in CARIS’s HIPS and SIPS software to 

retain only the soundings greater than 1 m and less than 2 m from nadir (the soundings across each 

swath with the highest precision). For the April 2013 survey of the Alviso tidal flats, the POS-derived 

bathymetry was, on average, 15 cm (SD = 3) deeper than the F180-derived bathymetry. There was a 

similar offset in the November 2013 surveys of the Dumbarton tidal flats, where the POS-derived 

bathymetry was an average of 11 cm (SD = 3) deeper than the F180-derived bathymetry. The source of 

the offset is unknown, but the similarity in magnitude and direction on the two separate surveys

suggests that it is a real bias. Data from the POS surveys were shoaled by the average offset of 13 cm

to keep the vertical reference frame consistent with the 2010 baseline bathymetric survey.

Sound Velocity Measurements

Sound velocity measurements were collected continuously with an Applied Micro Systems 

Micro SV (accurate to ±0.03 m/s) deployed on the transducer frame for real-time sound velocity 

adjustments at the transducer/water interface. Additionally, sound-velocity profile measurements of 

the water column were collected at least once per day. Sound-velocity profile measurements were 

collected using an Applied Micro Systems SvPlus 3472 which provides time-of-flight sound-velocity 

measurements using invar rods with a sound-velocity accuracy of ±0.06 m/s, pressure measured by a 

semiconductor bridge strain gauge to an accuracy to 0.15 percent (full scale), and temperature 

measured by thermistor to an accuracy of 0.05 degrees Celsius (Applied Microsystems, Ltd., 2005).

Processing Procedures

The general processing workflow for converting raw bathymetric soundings to a DEM is shown 

in figure 4. Critical aspects of the processing procedure are discussed in more detail below.
6

Figure 4. Processing workflow diagram for converting soundings to a digital elevation model.

Real-Time Sonar Sounding Processing

GPS data and measurements of vessel motion are combined in the F180 or POS MV hardware 

to produce a high-precision vessel attitude packet. This packet is transmitted to the Swath Processor 

acquisition software and combined with instantaneous sound velocity measurements at the 

transducer head before each ping. As many as 20 pings per second are transmitted, with each ping 

consisting of 2,048 samples per side (port and starboard). The returned samples are projected to the 

seafloor using a ray-tracing algorithm working with the previously measured sound velocity profiles in 

SEA Swath Processor. A series of statistical filters are applied to the raw samples that isolate the 

seafloor returns from other uninteresting targets in the water column. Finally, the processed data are

stored line-by-line in both raw (.sxr) and processed (.sxp) trackline files. When using the POS MV,

temporary sxp files are generated using attitude derived from tide predictions in real time, and final 
7

sxp files are generated during post processing when the POS SBET files are incorporated back into 

Swath Processor. For all of the Alviso surveys, processed files were filtered across-track with a mean 

filter at 0.2 m resolution.

Backscatter Image Production 

The relative differences in bay-floor backscatter strength (that is, the amplitude of the acoustic 

signal that is reflected back to the sonar head) can be a valuable tool for identifying changes in texture 

or composition of bay floor sediments. The raw 16-bit backscatter that is recorded simultaneously with 

the bathymetry by the SWATHplus was georeferenced and gain-normalized by the program SXPEGN 

(build 151) by David Finlayson (USGS) to enhance the backscatter of the SWATHplus system. The 

program normalizes for time-varying signal loss and beam directivity differences. The resulting 

normalized amplitude values are rescaled to 16-bit and gridded in Surfer (version 10) as shown in 

figure 5.

Figure 5. Map of backscatter amplitude (raw 16-bit digital number, DN), south San Francisco Bay, California.

Swath Cleaning and Filtering

Prior to creating the bathymetry grids, the processed .sxp files were imported to CARIS’s HIPS 

and SIPS software for additional cleaning and filtering. Swath filters were applied to clean the data

based upon characteristics such as depth, across track angle, and across track distance. A CARIS Swath 
8

Angle BASE (Bathymetric with Associated Statistical Error) surface was created at 1 m resolution and 

the subset editor was used to eliminate any remaining outliers or artifacts manually. The average 

depth within each 1 × 1 m cell was exported as an ASCII text file along with calculations of the binned

(of all soundings within the 1 × 1 m cell spacing), standard deviation, and sounding density. These ASCII 

files were used for statistical analyses and imported into Surfer (version 10) for gridding.

Digital Elevation Model Production

Bathymetric Grids

The 1 m resolution ASCII data were imported to Surfer (version 10) for statistical analysis and 

DEM generation. The binned ASCII data were interpolated in Surfer using a linear kriging algorithm 

with a 1-sigma nugget of 0.05 m (the mean standard deviation of all 1 × 1 m cells in the dataset) and a 

2 × 2 m search radius. This process filled small gaps in the surface and provided some minor smoothing 

through the statistical noise inherent to interferometric bathymetry. The 1 m resolution bathymetry

grid was exported to ESRI ArcMap software for display purposes (fig. 6) and converted from NAVD88

to the tidal datum of mean lower low water (MLLW) using the conversions generated by the National 

Oceanic and Atmospheric Administration (NOAA) and provided in Foxgrover and others (2007).

Changes in the elevation of the bay floor over time can be determined by simply differencing surveys 

(fig. 7). 

Figure 6. The 1 m resolution 2010 bathymetry grid generated by merging surveys S-2-10-SF, S-18-10-SF and S24-10-SF. Elevation in meters relative to NAVD88. 
9

Figure 7. Bathymetric change from 2010 to October 2016.

2010 Lidar Data

In 2010 the USGS, with funding provided by the American Recovery and Reinvestment Act 

(ARRA), contracted the acquisition of aerial topographic lidar in the San Francisco Estuary that included 

our study area. Data for the USGS San Francisco Coastal Lidar project were collected by Terrapoint 

USA, and Dewberry served as the primary contractor for the project and was responsible for all final 

data post-processing and classification necessary to develop project deliverables. The USGS aerial lidar 

was collected between June 11 and November 7, 2010, using a Piper Navajo twin engine aircraft 

equipped with a 100 kHz Optech ALTM 3100EA lidar system. Acquisition was designed to support a 

nominal point spacing of 1 m and was collected at low tide to optimize coverage of the intertidal flats.

Lidar data was processed by Dewberry to achieve a bare earth ground surface and provided as 2 m 

resolution hydro-flattened DEMs. When Dewberry compared the lidar to survey-grade GPS points in 

generally flat, nonvegetated areas, the vertical accuracy of 95 percent of the positions had errors less 

than or equal to 18 cm (equivalent to a 9 cm RMSE if evenly distributed). The lidar is projected in UTM 

coordinate space, zone 10 north. The vertical datum is NAVD88 and the horizontal datum 

NAD83(NSRS2007). Within our study area NAD83(NSRS2007) closely approximates the horizontal 

datum of the bathymetric data, NAD83(CORS96), and is within the accuracy of the data. For our 

purposes, the two versions of NAD83 are considered equivalent. For additional information on USGS’s 

ARRA San Francisco Coastal Lidar project or to download the data directly, visit 

https://earthexplorer.usgs.gov/ or http://www.coast.noaa.gov/dataviewer.
10

2010 Bathymetric / Topographic DEM

Prior to merging the 2010 bathymetry data with the topographic lidar data, a comparison was 

made between elevations of the two independent datasets where they overlap in the intertidal flats.

There is approximately 1.5 km2 of overlap between lidar and bathymetry data within the study area 

(fig. 8). For all areas of overlap, the lidar is a maximum of 3.3 m higher than or 1.2 m lower than the 

bathymetry in the region of overlap. The average difference is 0.05 m (SD=0.30). Upon close 

examination, it became apparent that large differences between the two surfaces (greater than 1 m)

occurred along very narrow strips of channel margins or at two specific sites: (1) along Coyote Creek 

(fig. 8B), and (2) along Alviso Slough south of UTM northing 4144000 (fig. 8C). At these two locations 

the lidar elevations are more than 1 m higher than the bathymetry elevations and are an artifact of the 

lidar reflecting off the water surface, not the bay floor. These areas were removed from the lidar DEM 

and excluded from further analyses. When the difference analyses were restricted to the intertidal flat 

areas adjacent to the confluence of Guadalupe Slough and Coyote Creek (fig. 8A) the statistics greatly 

improved. In the intertidal flats the lidar is a maximum of 1.2 m higher than or 0.8 m lower than the 

bathymetry. The mean offset on the tidal flats is 0 m (SD=0.07), which suggests that there is not a 

consistent offset or bias between the two and serves as an independent check on the quality of the 

two datasets.

Prior to merging the surfaces, the bathymetry was resampled to 2 m resolution to match the

lidar DEM by using a bilinear interpolation. The bathymetry was then merged with the topographic 

lidar by using the blend algorithm in the “Mosaic to New Raster” tool in Arc Toolbox. The resultant 

DEM is provided at 2 m resolution with elevations relative to NAVD88 (fig. 9). To provide intertidal 

elevations relative to the tidal datum of MLLW, the DEM was clipped to the approximate extent of the 

shoreline (modified from the San Francisco Estuary Institute EcoAtlas [1998] modern baylands 

shoreline) and converted to MLLW by using the NAVD88 to MLLW conversions calculated by NOAA and 

provided by Foxgrover and others (2007; fig. 10).
11

Figure 8. Difference between tidal flat elevations calculated from 2010 aerial lidar data versus bathymetry data, 

south San Francisco Bay, California.
12

Figure 9. The 2010 seamless bathymetric/topographic DEM of the region surrounding Coyote Creek and Alviso 

Slough, south San Francisco Bay, California. Elevations relative to NAVD88.

Figure 10. The 2010 seamless bathymetric/topographic DEM of Coyote Creek and Alviso Slough clipped to the 

approximate extent of the shoreline, south San Francisco Bay, California. Elevations relative to MLLW.
13

Horizontal and Vertical Datum Conversions

To meet the varying needs of the end users of these data products the surfaces are provided 

relative to orthometric heights NAD83(CORS96)/NAVD88, the ellipsoid WGS84(G1150), and relative to 

the tidal datum of MLLW. V-Datum (version 2.3.3) does a poor job of converting between geodetic and 

tidal datums in far south San Francisco Bay. Some investigations have revealed that benchmark 

information for the Dumbarton Bridge Station (station 9414509) was not used in development of the 

VDatum model. Due to the lack of model constraint in this region, VDatum consistently 

underestimates the offset between NAVD88 and MLLW for regions south of the Dumbarton Bridge. A 

comparison of conversions from NAVD88 to MLLW generated by VDatum to those provided by the COOPS division of NOAA for a 2005 bathymetric survey of south San Francisco Bay (Foxgrover and others, 

2007) reveals a difference between the two of approximately 17 cm near Dumbarton Bridge, 

approximately 20 cm where Guadalupe Slough meets Coyote Creek, and greater than 30 cm near the 

island ponds. The conversion to MLLW used for this report is based upon the CO-OPS data provided by

Foxgrover and others (2007), and we caution that using VDatum for converting between geodetic and

tidal datums south of Dumbarton Bridge could introduce errors on the order of tens of centimeters.

To convert the data from orthometric heights NAD83(CORS96)/NAVD88 to ellipsoid heights on

ITRF2000 (also known as WGS84 G1150), the Surfer grids were exported as ASCII tables. NOAA’s 

VDatum version 2.3.0 (http://vdatum.noaa.gov) was used to apply Geoid09 (National Geodetic Survey, 

2009) and convert the orthometric height data into ellipsoid heights on the NAD83(CORS96) ellipsoid. 

The data were next transformed from the NAD83 (CORS96) ellipsoid to the ITRF2000 ellipsoid by using 

a 14-point Helmert transformation described by Soler and Snay (2004) using the command line tool 

CS2CS in the Proj4 library (http://trac.osgeo.org/proj/). The parameters were calculated for an Epoch 

date of 2007.0000 as shown in table 4. The conversion from NAD83(CORS96) to ITRF2000 shifts the 

Easting coordinates of the survey by approximately –1.3 m, the Northing coordinate by approximately 

+0.40, and the z coordinate by about –0.54 m. To convert the POS-derived bathymetry from ellipsoid 

heights on WGS84 to orthometric heights NAD83(CORS96)/NAVD88, the same CS2CS transformation 

was applied but in reverse, and rather than using VDatum, a fixed Geoid09 offset of –32.63 m from our 

base station detailed in table 3 was applied.

Table 4. Parameters adopted for transformation between NAD83(CORS96) and ITRF2000

Parameter Definition Units Value at t0 = 1997.0 Value at tF = 2007.0 

Tx x-shift meters 0.9956 1.0026

Ty y-shift meters -1.9013 -1.9083

Tz z-shift meters -0.5215 -0.5165

ωx x-rotation1 arc seconds -0.025915 -0.026585

ωy y-rotation1 arc seconds -0.009426 -0.001856

ωz z-rotation1 arc seconds -0.011599 -0.011089

s scale parts-per-million 0.00062 -0.00118

1

Note that the Proj4 program cs2cs reverses the sign of the rotation parameters from the Soler and Snay (2004) algorithm. 

Because the transformation here is from NAD83 to ITRF2000, the program is run in reverse-mode (-I).
14

Estimates of Bathymetric Uncertainty

There are multiple techniques for assessing the uncertainty of bathymetric surveys. For 

relatively flat portions of the seafloor, the standard deviation of sounding elevations within a small

area is a good measure of the precision of the sonar instrumentation; this is not true in areas where 

the seafloor is naturally variable or steep. In these areas, the standard deviation reflects the 

combination of natural variation of the surface in addition to sonar measurement uncertainty.

A plan-view map of typical spatial variability in standard deviations throughout the study area 

is shown in figure 11. The overall spatial pattern of standard deviation reveals low standard deviation 

in the low-relief intertidal flats and increases with increasing depth and slope (that is, in and along 

channel banks). The mean standard deviation of the 2010 soundings within each 1 × 1 m cell 

(containing 19 soundings on average) for the entire study area is 0.05 m, and 97 percent of the cells 

have a standard deviation less than 0.15 m (fig. 12). The mean standard deviation over the intertidal 

flats for each of the thirteen surveys collected between 2010 and 2017 varies from 2–5 cm, with the 

exception of the April 2013 survey, which has a mean standard deviation of 7 cm. In general, the 

coverage of the intertidal flats during our spring surveys is severely limited due to unfavorable tidal 

conditions. The fall surveys, when favorable tidal conditions allow greater intertidal flat coverage, 

provide a more robust statistical measure of survey precision. However, since a large portion of our 

study area is comprised of channels, the overall mean standard deviation alone does not provide an

adequate approximation of survey precision.
15

Figure 11. Plan-view map of bathymetric soundings standard deviation within each 1 × 1 m cell, south San 

Francisco Bay, California.

Figure 12. Histogram of sounding elevation standard deviations within each 1 × 1 m cell.

A second technique for assessing the precision of bathymetric data is to compare the elevation 

at a given location as calculated from data collected on independent tracklines that intersect. For this 

analysis a number of survey tracklines were collected on the intertidal flats running in an approximate 

northwest/southeast direction that is nearly perpendicular to the primary trackline orientation for this 

region. For the 2010 survey, these intersecting tracklines (also known as tie lines) result in 

approximately 100 intersections which are used for statistical analyses. For the purposes of error 

assessment, the tie lines were aggressively trimmed in CARIS’s HIPS and SIPS software to retain only 

the soundings greater than 1 m and less than 2 m from nadir (the soundings across each swath with 

the highest precision). The surface generated from these tie lines was differenced from the 

bathymetric surface generated from the northeast/southwest oriented primary survey tracklines to 

see how the two compared. For the 2010, February 2012, and April 2012 surveys the mean difference 

between the tie line intersections was 1 cm and the standard deviation 3 cm, which suggests a vertical 

uncertainty of approximately ±5 cm in the intertidal flats. Tie line analyses for POS-derived 

bathymetric surveys collected in 2013, 2014, April 2015, and April 2016 revealed comparable mean 

differences of 2 cm or less with standard deviations of 3–5 cm.

The mean difference from tie line analyses of the fall surveys of 2011, 2012, 2015, and 2016

were slightly elevated and warranted further examination. It was determined during the October 2011 

survey that the configuration offset between the IMU and the primary GPS antenna was accidentally 

inverted during the first four days of surveying. As a result, the IMU was unable to properly calibrate

its position. Fortunately, eight tie lines were collected throughout the western and eastern tidal flats 

during the last three days of this survey when the IMU was functioning properly. The extensive 

coverage of the tie lines enabled us to evaluate offsets due to the IMU problems and shoal the survey 
16

lines by the appropriate amount (varying from 8 to 33 cm) to minimize offsets. After performing these 

corrections the mean difference between tie line intersections dropped to 2 cm (SD=3).

During post-processing of the October 2012 survey tie line analyses revealed a depth-bias in 

lines collected on the western intertidal flats on October 19th. The cause of this bias is uncertain; 

however it may be the result of an imprecise IMU position calibration on that day. These intertidal flat 

survey lines collected on October 19th were shoaled by 8 cm to minimize offsets between tie lines.

After the shift was applied the mean difference of tie line intersections for this survey was 2 cm 

(SD=4). Note that Guadalupe Slough was also surveyed on October 19th, but data within the slough did 

not appear to show the same offset as soundings within the flats, and as a result, were not adjusted.

The spatial extent of survey lines that were manually adjusted for both the October 2011 and October 

2012 surveys are delineated in a polygon shapefile that is provided with the bathymetric grids. The 

accuracy of elevations within these polygons is less reliable than for the remainder of these datasets.

The October 2015 and October 2016 surveys have mean tie-line differences of 7 cm (SD=4) and 

6 cm (SD=6), respectively. There was no discernable bias by survey day and we are uncertain of the 

cause of the decline in survey precision. The SWATHplus system was modified prior to the October 

2015 survey to incorporate a third, forward-looking transducer to fill the nadir gap beneath the survey 

vessel in an attempt to decrease surveying time. Although data from the additional transducer was not 

included in our post-processing due to software limitations, it is possible that modifications made 

when the third transducer was added, either within the system itself, or within processing software, 

could be introducing additional noise in the data. The third transducer was also collecting data during 

the spring 2016 and 2017 surveys, which show mean differences of 1–2 cm (SD=3), values consistent 

with earlier surveys collected using only two transducers. However, intertidal flat coverage during 

these spring surveys was extremely limited, making it difficult to perform a robust assessment of 

survey precision.

Vertical Adjustments to Post-2010 Surveys

Over the years various improvements have been made to our bathymetric mapping system. For 

the purposes of bathymetric change detection, it is important to keep all of our surveys as consistent 

with the 2010 baseline survey as possible. Every effort has been made to ensure that difference maps 

between any of the Alviso surveys reflect actual changes of the bay floor and are not simply a 

reflection of changes to our survey system. The following uniform vertical shifts have been applied to 

post-2010 surveys to adjust them to the vertical reference plane of the 2010 baseline survey. All of the 

post-2010 surveys have been deepened by 6 cm to account for a recalculation of the transducer leverarm offsets that was performed in 2011. All of the surveys collected since April 2013 using the POS MV

IMU, were also shoaled by 13 cm to account for the measured F180/POS MV bias. All surveys collected 

since October 2015, when the forward-looking transducer was added, have been deepened by 3 cm to 

account for transducer mounting plate modifications and more precise lever arm to IMU 

measurements. Surveys collected in March 2017 using the upgraded POS MV V5 have been shoaled by 

an additional 7 cm to account for an IMU reference-point offset that had inadvertently shoal biased all 

of the surveys collected with the POS MV V4. The cumulative effects of changes due to either 

modifications to the survey platform, or changes within software settings have resulted in the net 

vertical adjustments listed in table 5.
17

Table 5. Cruise dates, type of Inertial Measurement Unit (IMU) used, number of SWATHplus transducers 

collecting data, and net vertical adjustment applied to align surveys with the same vertical reference plane as the 

2010 baseline survey. 

Cruise 

Dates

IMU used Number of 

Transducers

Net Vertical Adjustment to 2010 

Baseline

Jan 2010 F180 2 0

Sept 2010 F180 2 0

Dec 2010 F180 2 0

Oct 2011 F180 2 −6 cm

Feb 2012 F180 2 −6 cm

Apr 2012 F180 2 −6 cm

Oct 2012 F180 2 −6 cm

Apr 2013 POS MV V4 Model 320 2 +7cm

Nov 2013 POS MV V4 Model 320 2 +7cm

Oct 2014 POS MV V4 Model 320 2 +7cm

Apr 2015 POS MV V4 Model 320 2 +7cm

Oct 2015 POS MV V4 Model 320 3 +4cm

Apr 2016 POS MV V4 Model 320 3 +4cm

Oct 2016 POS MV V4 Model 320 3 +4cm

Mar 2017 POS MV V5 Model 320 3 +11cm

Data Tables

Bathymetry

Bathymetry data are provided as elevation in meters relative to both the ellipsoid 

WGS84(G1150) and as orthometric heights NAD83(CORS96)/NAVD88, as well as relative to the tidal 

datum of MLLW (table 5). All data are projected in UTM, zone 10 north, and all values (eastings, 

northings, and elevation) are in meters. Each zip file contains the data formatted as both ASCII X, Y, Z 

text files (*.xyz) and ESRI ASCII files (*.asc; see appendix A), as well as FGDC compliant metadata in 

both text and .xml format.

Table 6. Bathymetric data files provided.

File name Horizontal resolution,

in meters

Reference frame Vertical datum

2010_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

2010_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

2010_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Oct11_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Oct11_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Oct11_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Feb12_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Feb12_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Feb12_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Apr12_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Apr12_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Apr12_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Oct12_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Oct12_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Oct12_bathy_MLLW.zip 1 NAD83(CORS96) MLLW
18

File name Horizontal resolution,

in meters

Reference frame Vertical datum

Apr13_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Apr13_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Apr13_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Nov13_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Nov13_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Nov13_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Oct14_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Oct14_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Oct14_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Apr15_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Apr15_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Apr15_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Oct15_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Oct15_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Oct15_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Apr16_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Apr16_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Apr16_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Oct16_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Oct16_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Oct16_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

Mar17_bathy_NAVD88.zip 1 NAD83(CORS96) NAVD88

Mar17_bathy_WGS84.zip 1 WGS84(G1150) WGS84(G1150)

Mar17_bathy_MLLW.zip 1 NAD83(CORS96) MLLW

2010 Bathymetric / Topographic DEM

Seamless bathymetric/topographic DEMs generated from merging the 2010 bathymetry 

(above) with aerial lidar are provided as elevations in meters relative to both the ellipsoid 

WGS84(G1150) and as orthometric heights NAD83(CORS96)/NAVD88 (table 6). The merged DEM was 

clipped to extent of the shoreline and converted from the geodetic vertical datum of NAVD88 to the 

tidal datum of MLLW based upon the conversions calculated by NOAA and provided in Foxgrover and 

others (2007). All data are projected in UTM, zone 10 north and all values (eastings, northings, and 

elevation) are in meters. Each zip file contains the data formatted as both ASCII X, Y, Z text files (*.xyz) 

and ESRI ASCII files (*.asc; see appendix A) as well as FGDC compliant metadata in both text and .xml 

format.

Table 7. Bathymetric/topographic DEM files provided.

File name Horizontal resolution, in meters Reference frame Vertical datum

2010_DEM_NAVD88.zip 2 NAD83(CORS96) NAVD88

2010_DEM_WGS84.zip 2 WGS84(G1150) WGS84(G1150)
19

File name Horizontal resolution, in meters Reference frame Vertical datum

2010_DEM_bay_NAVD88.zip 2 NAD83(CORS96) NAVD88

2010_DEM_bay_WGS84.zip 2 WGS84(G1150) WGS84(G1150)

2010_DEM_bay_MLLW.zip 2 NAD83(CORS96) MLLW

A Note on Coordinate Systems and Datums

WGS84 and NAD83 have been revised several times resulting in coordinate shifts of as much as

several meters in the X, Y, and Z directions. The revision is indicated by the designator following the 

name (G1150 following WGS84, for example). Software that does not distinguish between the 

different versions of these datums likely does not support 3D datums properly. Users should pay 

particular attention to the accompanying metadata files to ensure that the data are properly 

georeferenced. In particular, note that most current GIS software (including ArcGIS 10) cannot 

properly transform high-resolution elevation data from one 3D datum to another (such as WGS84 

G1150 to NAD83 CORS96) without introducing errors on the order of 1–2 m in X, Y, and Z directions.

For this reason, data are provided in both WGS84(G1150), which is equivalent to ITRF2000, and 

NAD83(CORS96), which is equivalent to NSRS2007.

Acknowledgments

Thanks to the survey crew of the R/V Parke Snavely: Mike Boyle, Jamie Grover, Greg Gabel, 

Jennifer White, Pete Dal Ferro, Jackson Currie, Gerald Hatcher, Rob Wyland, Daniel Powers, and Rachel 

Marcuson (all of the USGS) who assisted with the bathymetry collection and to Tom Reiss who 

established GPS base stations for the surveys. Funding was provided by the California Coastal 

Conservancy, U.S. Environmental Protection Agency, the Resources Legacy Fund, Santa Clara Valley 

Water District, and the USGS Coastal and Marine Geology Program.

References Cited

Applied Microsystems Ltd., 2005, SVplus sound velocity, temperature, and depth profiler—User’s 

manual (revision 1.23): Applied Microsystems Ltd., 39p.

Foxgrover, A.C., Jaffe, B.E., Hovis, G.T., Martin, C.A., Hubbard, J.R., Samant, M.R., and Sullivan, S.M., 

2007, 2005 Hydrographic survey of south San Francisco Bay, California: U.S. Geological Survey OpenFile Report 2007–1169, 113 p., accessed November 15, 2011, at

http://pubs.usgs.gov/of/2007/1169/.

National Geodetic Survey, 2009, GEOID09 geoid model: National Oceanic and Atmospheric 

Administration, accessed May 1, 2015, at http://www.ngs.noaa.gov/GEOID/GEOID09/.

San Francisco Estuary Institute, 1998, EcoAtlas (version 1.50b4): accessed May 1, 2015, at

http://www.sfei.org/content/ecoatlas-version-150b4-1998. [Also available on CD-ROM.]

Systems Engineering and Assessment, Ltd., 2004, SWATHplus training pack: Systems Engineering and 

Assessment, Ltd.

Soler, T., and Snay, R.A., 2004, Transforming positions and velocities between the International 

Terrestrial Reference Frame of 2000 and North American Datum of 1983: Journal of Surveying 

Engineering, v. 130, no.2, p. 49–55.
20

Appendix A

National Geodetic Survey Datasheet for Monument ARC 34 (PID DG6881) Used as geodetic control for 

the Alviso Survey 

DATABASE = ,PROGRAM = datasheet, VERSION = 7.85 

1 National Geodetic Survey, Retrieval Date = JUNE 8, 2010

DG6881 

*********************************************************************

**

DG6881 HT_MOD - This is a Height Modernization Survey Station.

DG6881 DESIGNATION - ARC 34

DG6881 PID - DG6881

DG6881 STATE/COUNTY- CA/SANTA CLARA

DG6881 USGS QUAD - MOUNTAIN VIEW (1997)

DG6881

DG6881 *CURRENT SURVEY CONTROL

DG6881 

___________________________________________________________________

DG6881* NAD 83(2007)- 37 25 34.57880(N) 122 02 05.53373(W) ADJUSTED

DG6881* NAVD 88 - 1.28 (meters) 4.2 (feet) GPS OBS

DG6881 

___________________________________________________________________

DG6881 EPOCH DATE - 2007.00

DG6881 X - -2,690,026.780 (meters) COMP

DG6881 Y - -4,299,118.359 (meters) COMP

DG6881 Z - 3,855,050.006 (meters) COMP

DG6881 LAPLACE CORR- 0.35 (seconds) DEFLEC09

DG6881 ELLIP HEIGHT- -31.308 (meters) (02/10/07) ADJUSTED

DG6881 GEOID HEIGHT- -32.62 (meters) GEOID09

DG6881

DG6881 ------- Accuracy Estimates (at 95 percent Confidence Level in 

cm) --------

DG6881 Type PID Designation North East Ellip

DG6881 --------------------------------------------------------------

-----

DG6881 NETWORK DG6881 ARC 34 0.39 0.35 1.02

DG6881 --------------------------------------------------------------

-----

DG6881

DG6881.The horizontal coordinates were established by GPS 

observations

DG6881.and adjusted by the National Geodetic Survey in February 2007.

DG6881

DG6881.The datum tag of NAD 83(2007) is equivalent to NAD 

83(NSRS2007).
21

DG6881.See National Readjustment for more information.

DG6881.The horizontal coordinates are valid at the epoch date 

displayed above.

DG6881.The epoch date for horizontal control is a decimal equivalence

DG6881.of Year/Month/Day.

DG6881

DG6881.The orthometric height was determined by GPS observations and 

a

DG6881.high-resolution geoid model using precise GPS observation and

DG6881.processing techniques.

DG6881

DG6881.The X, Y, and Z were computed from the position and the 

ellipsoidal ht.

DG6881

DG6881.The Laplace correction was computed from DEFLEC09 derived 

deflections.

DG6881

DG6881.The ellipsoidal height was determined by GPS observations

DG6881.and is referenced to NAD 83.

DG6881

DG6881.The geoid height was determined by GEOID09.

DG6881

DG6881; North East Units Scale Factor Converg.

DG6881;SPC CA 3 - 603,912.284 1,864,158.605 MT 0.99994515 -0 56 22.9

DG6881;SPC CA 3 - 1,981,335.55 6,115,993.69 sFT 0.99994515 -0 56 22.9

DG6881;UTM 10 - 4,142,598.916 585,392.741 MT 0.99968982 +0 35 11.7

DG6881

DG6881! - Elev Factor x Scale Factor = Combined Factor

DG6881!SPC CA 3 - 1.00000491 x 0.99994515 = 0.99995006

DG6881!UTM 10 - 1.00000491 x 0.99968982 = 0.99969473

DG6881

DG6881 SUPERSEDED SURVEY CONTROL

DG6881

DG6881 NAD 83(1998)- 37 25 34.57542(N) 122 02 05.53012(W) AD(2002.75) 

B

DG6881 ELLIP H (08/23/04) -31.247 (m) GP( ) 4 1

DG6881

DG6881.Superseded values are not recommended for survey control.

DG6881.NGS no longer adjusts projects to the NAD 27 or NGVD 29 

datums.

DG6881.See file dsdata.txt to determine how the superseded data were 

derived.

DG6881

DG6881_U.S. NATIONAL GRID SPATIAL ADDRESS: 10SEG8539242598(NAD 83)

DG6881_MARKER: DD = SURVEY DISK

DG6881_SETTING: 7 = SET IN TOP OF CONCRETE MONUMENT

DG6881_STAMPING: ARC 34
22

DG6881_STABILITY: C = MAY HOLD, BUT OF TYPE COMMONLY SUBJECT TO

DG6881+STABILITY: SURFACE MOTION

DG6881_SATELLITE: THE SITE LOCATION WAS REPORTED AS SUITABLE FOR

DG6881+SATELLITE: SATELLITE OBSERVATIONS - September , 2002

DG6881

DG6881 HISTORY - Date Condition Report By

DG6881 HISTORY - UNK MONUMENTED NASA

DG6881 HISTORY - 200209 GOOD JOHFRA

DG6881

DG6881 STATION DESCRIPTION

DG6881

DG6881'DESCRIBED BY JOHNSON-FRANK 2002 (RAF)

DG6881'THE STATION IS ON MOFFETT AIRFIELD, NEAR MOUNTAIN VIEW. FROM 

THE

DG6881'INTERSECTION OF HWY 101 AND ELLIS ST (SE OF HWY 85), EXIT 

NORTH TO

DG6881'THE ENTRANCE TO MOFFETT FIELD. AFTER GOING THROUGH THE 

GUARDPOST,

DG6881'CONTINUE AHEAD TO SIGNAL LIGHT. TURN RIGHT ON PERIMETER RD

DG6881'(LOCALLY-CALLED) AND DRIVE 0.7 MI PARALLELING HWY 101 UNTIL 

THE SHARP

DG6881'BEND TO THE LEFT. FOLLOW THE ROAD AND DRIVE 1.3 MI TO A SIDE 

ROAD

DG6881'RIGHT AND A GOLF COURSE. TURN RIGHT AND DRIVE 0.2 MI TO AN

DG6881'INTERSECTION, THEN LEFT FOR 0.5 MI TO THE STATION ON THE RIGHT 

JUST

DG6881'BEFORE THE ROAD MAKES A NINETY-DEGREE TURN TO THE LEFT AT THE 

VERY

DG6881'NORTHEAST CORNER OF THE MOFFETT FIELD FACILITY. THE MARK IS 

ABOUT 23

DG6881'M (75 FT) SOUTH OF A CHAIN LINK GATE.

DG6881'

DG6881'MARK IS AN 8.2 CM (3.25 IN) NATIONAL AERONAUTICS AND SPACE

DG6881'ADMINISTRATION DISK STAMPED 'ARC 34' WITH A PUNCH NEXT TO THE 

'34'.

DG6881'THE DISK IS SET IN AN IRREGULAR CONCRETE MASS FLUSH WITH THE 

SOIL,

DG6881'6.7 M (22 FT) EAST OF THE CENTER OF THE PAVED PERIMETER ROAD, 

22.8 M

DG6881'(75 FT) SOUTH OF A CHAIN LINK FENCE GATE AT THE END OF THE 

ROAD, 4.3

DG6881'M (14 FT) SOUTH OF THE CONCRETE BASE FOR A SMALL SQUARE WHITE 

TANK.

DG6881'

DG6881'THIS STATION WAS OBSERVED AS PART OF THE SOUTH SAN FRANCISCO 

BAY

DG6881'HEIGHT MODERNIZATION PROJECT.
23

*** retrieval complete.

Elapsed Time = 00:00:00
24

Appendix B

Description of the ESRI ASCII raster format:

To import ASCII files into common GIS packages:

ArcGIS: Use ArcTools's ASCII to Raster function

ArcView: Use the import ASCII Grid function (May need Spatial Analyst)

GRASS: Use the 'r.in.arc' function.

The ASCII file consists of header information containing a set of keywords, followed by cell values in 

row-major order. The file format is:

<NCOLS xxx>

<NROWS xxx>

<XLLCENTER xxx | XLLCORNER xxx>

<YLLCENTER xxx | YLLCORNER xxx>

<CELLSIZE xxx>

{NODATA_VALUE xxx}

row 1

row 2

.

.

.

row n

where xxx is a number, and the keyword nodata_value is optional and defaults to -9999. Row 1 of the 

data is at the top of the grid, row 2 is just under row 1 and so on. The nodata_value is the value in the 

ASCII file to be assigned to those cells whose true value is unknown. In the grid they will be assigned 

the keyword NODATA. Cell values are delimited by spaces. No carriage returns are necessary at the 

end of each row in the grid (although they are included in this case). The number of columns in the 

header is used to determine when a new row begins. The number of cell values is equal to the number 

of rows times the number of columns.