# Presentation: 657661

## Geospatial Accuracy Issues

- Rick Koehler, PhD
- NOAA – NWS – OCWWS – FDTB
- Boulder, CO

- RFC GIS workshop
- OHRFC
- Wilmington, OH
- July 17, 2007

*richard.koehler@noaa.gov*

## Outline

- Accuracy needs assessment
- Error sources
- Geospatial data
- 	Map datums and projections
- 	Accuracy of NWS GIS datasets
- WSR-88D study
- Review
- Summary hydrology assessment

**Page  2**

*39° 25' 13” N*
*83° 49' 18” W*
*(NAD83)*

## Accuracy Needs Assessment

- Hydrologic modeling error sources
- 		Model, Parameters, Input data
- Data types
- 	Precip, Soil moisture, Temperature, Met, Streamflow
*	**Gridded*
*	**High resolution*
*	**Four (4) km scale*
*	**High quality*
*Unstated assumption:*
*	**	**Data are positioned correctly*

**Page  3**

## Accuracy Needs Assessment

- Geospatial element
- 	Gridded...4 km scale...high resolution = geospatial
- 	“Observation” = measured environ. attribute at a location
- 	“Forecast” = predicted environ. attribute at a location
- 	Need to QC/QA met, hydro, _and_ geospatial data

**Page  4**

## Accuracy Needs Assessment

- Geospatial questions to ask before a project
- 	What are your project needs?
* *
- 	General rule of thumb –
*Accuracy of geospatial data ~10x more accurate than project*
*Don’t want data position accuracy to affect project*
- What geospatial data are available?
- 	What errors and uncertainties exist in this data?
- 	Does this information meet your accuracy needs?

**Page  5**

## Accuracy Needs Assessment

- Accuracy vs Precision confusion
- 	River stage:  	15.123942 ft (± 0.01 ft)
- 					vs
- 				15.12 ft (± 0.01 ft)
- 	Position:       35.129812° N (± 0.01°)
- 			     109.201923° W (± 0.01°)
- 			*        **(NAD83)*

**Page  6**

## Error Sources in GIS

- Error sources
- 	Editing, input, digitizing hardware, techniques
*	**(lineage, positional accuracy, attribute accuracy,     **	**logical consistency, and completeness)*
- Primary sources of error in GIS data
-   	Human
- 	Systematic
- 	Random
- Positional Accuracy
- 	Does location match reality?

**Page  7**

## Example

**Google Maps**

**Page  8**

## Geospatial data: experiment

- Find the latitude and longitude of the flagpole

- University Corporation for Atmospheric Research (UCAR)

- Unit has several different “map datums” built into the software
*(new ones have 70+ datums)*

**Page  9**

## Geospatial data: results

**Page  10**

## Geospatial data: key terms

- Geoid - A reference level surface based on gravity.  Elevation is tied to the Geoid.

**Page  11**

## Geospatial data: key terms

- Reference ellipsoid or spheroid– an approximation of the geoid.
- A mathematically “smooth” surface for calculations. Impossible on an irregular bumpy surface.

**Geoid**
**“****bumpy” real Earth**

**Ellipsoid, spheroid**
**“****smooth” math Earth**

- Flattening = (a-b)/a  	
- Inverse flattening (f-1)	   300

**Page  12**

## Geospatial data: ellipsoid/spheroid

**Page  13**

## Geospatial data: datum

- Horizontal Datum –
- _THE_ link between the geoid and a reference ellipsoid

**Earth’s rotational axis**

**Ellipsoid semi-major axis**

**Geoid**

**Earth’s center**

**Ellipsoid**
**semi-minor**
**axis**

**Astro lat**

**Geodetic lat**

**Perpendicular to geoid and ellipsoid**

**Datum origin**

- (Note: Very “_Old School_” approach!)

**Page  14**

## Geospatial data: datum

- International 1924 		76
- Clarke 1880		44
**Clarke 1866****	****	****33**
- South American 1969 		13
- Krassovsky 1940 		8
- Bessel 1841		7
- Everest (India 1830) 		6
- Airy 1830 			5
- Bessel 1841 		3
- Australian National 		3
**GRS 80 ****	****	****	****1**
**WGS 84****	****	****	****1**
- WGS 72			1
- Modified Fischer 1960 		1
- Indonesian 1974 		1
- Hough 1960 		1
- Helmert 1906 		1
- Everest (Sabah Sarawak) 	1
- Everest (India 1956) 		1
**	**
**Total****	****	****	****200+  **

- In GIS-speak, a horizontal datum is a specific geographic coordinate system.
- Multiple datums exist.

**Page  15**

## Geospatial data: datums

**NAD27 vs NAD83 **
**CONUS apparent shift 5 to 100 meters, AK ~ 150+ m, HI ~ 400+ m **

- NOAA/NGS

- Geospatial data: datums

**Page  16**

## Geospatial data – legacy info

**Page  17**

## Geospatial data – why change

- www.photolib.noaa.gov/historic/c&gs/theb1581.htm

- www2.eng.cam.ac.uk/~alj3/Survey/4M9pics/

- 2007

*State of the art in surveying*

**Better equipment**
**Better techniques**
**Improved Earth model**
**Better survey network**
**More observed data**

- 1922

**Page  18**

## National Geodetic Survey states for NAD 27:
	Data no longer accepted
	Reference ellipsoid center is not geocentric (Clarke 1866)
			GPS data does not fit well into NAD 27
	Has significant distortion
	Is poorly connected to the geoid (elevation related issues)
	Is orders of magnitude less accurate than NAD83
			NAD 27 data mainly from the 1800’s and early 1900’s

NAD 27 is obsolete
	NAD 27 is no longer the official horizontal datum of the US govt.
	Federal Register Notice, Vol. 54 No. 113/, Wednesday 6/14/89

- 	Data no longer accepted
- 	Reference ellipsoid center is not geocentric (Clarke 1866)
- 			*GPS data does not fit well into NAD 27*
- 	Has significant distortion
- 	Is poorly connected to the geoid (elevation related issues)
- 	Is orders of magnitude less accurate than NAD83
*	**	**	**NAD 27 data mainly from the 1800’s and early 1900’s*
**NAD 27 is obsolete**
- 	NAD 27 is no longer the official horizontal datum of the US govt.
- 	Federal Register Notice, Vol. 54 No. 113/, Wednesday 6/14/89

- Geospatial data - changes

**Page  19**

## Geospatial data: NSRS

**Hydrology**
**Land use**
**Vegetation**

- Longitude

- Latitude

**Page  20**

## Error Sources in GIS:  WSR-88D

- These (ARC) versions use a version of the fsl-w88d shapefile _used in AWIPS_ to determine radar locations (nexradsites.*).

- http://www.weather.gov/geodata/utils/html/radardensity.htm

- http://mi3.ncdc.noaa.gov/mi3report/MISC/nexrad-stations.TXT

- A fixed-length text summary listing of _current_ NEXRAD radar stations, including identifiers and the _latest known location information._

**Page  21**

## Error Sources in GIS 

| Statistic | NCDC vs OST |
| --- | --- |
| No. WSR-88D pairs | 159 |
| No. pairs with exact same lat/long | 0 |
| Max difference | 5,500 m (5.5 km) |
| Avg difference | 85 m |
| Median difference | 33 m |
| # pairs diff by 100+ m | ~ 10% |
| # pairs diff by 20+ m | ~ 70% |
| NWS GIS data precision | 0.1 m (4 inches) |

**Page  22**

- !

## Error Sources in GIS

- NCDC	KIWX	FT WAYNE	41.40861 N	85.7 W
- OST	KIWX	North Webster	41.35890 N	85.70000 W

** **

-  = 5.5 km

**Page  23**

## Geospatial data - hydro metadata

|  | Source scale | Res (°) | Datum | Reference Ellipsoid | Semi-major axis (a) | Inv. flattening |
| --- | --- | --- | --- | --- | --- | --- |
| Basins | 1:2,000,000 | 0.00001 | NAD83 | GRS80 | 6,378,137 | 298.257 |
| Lakes | 1:2,000,000 | 0.000001 | NAD83 | GRS80 | 6,378,206.4 | 298.2572221 |
| Rivers | 1:2,000,000 | 0.000001 | NAD27 | Clarke 1866 | 6,378,206.4 | 294.978698 |
| RFC regions | 1:2,000,000 | 0.000001 | NAD83 | Clarke 1866 | 6,378,206.4 | 294.978698 |
| HAS regions | ? | 0.000001 | NAD83 | GRS80 | 6,378,206.4 | 294.978698 |
| Counties | 1:2,000,000 | 0.000001 | NAD83 | GRS80 | 6,378,206.4 | 294.978698 |
| States | 1:2,000,000 | 0.000001 | NAD83 | GRS80 | 6,378,137 | 298.257 |

- Correct datum, ellipsoid, semi-major axis, inv flattening
- NAD83	GRS80	      6,378,137             298.2572221
- NAD27	Clarke 1866      6,378,206.4          294.978698

**?**

**?**

**?**

**?**

**?**

**Page  24**

## Geospatial data: coordinates

- Geospatial data: coordinates

*Map projection coordinates *
_*Display*_* based*

*Geographic/geodetic coordinates*
_*Ground*_* based*

**Page  25**

## Coordinate precision 

- Distance for longitude 
- Latitude distance * cos latitude

- 1’ long  1’ lat * (cos lat)

| Latitude (°) | Distance (m) |
| --- | --- |
| 1°  (60 nmi) | 111,120. |
| 0.1° | 11,112. |
| 0.01° | 1,111.2 |
| 0.001° | 111.12 |
| 0.0001° | 11.11 |
| 0.00001° | 1.11 |
| 0.000001° | 0.11  (~ 4 in) |

| Latitude (° ‘ “) | Distance (m) |
| --- | --- |
| 1°  (60 nmi) | 111,120 |
| 1’   (1 nmi) | 1,852 |
| 1” | 31 |
| 0.1” | 3.1 |
| 0.01” | 0.3 (~ 1 ft) |

*NWS GIS data listed to this precision*

**Page  25**

## Reproject  in “GIS talk” = change the x,y of data

Typically based on geographic coordinates
Linked to a specific reference ellipsoid
Tied to a specific horizontal datum
Usually given as latitude and longitude

End up with projected coordinates 
Ultimately based on geographic coordinates
X,Y values will differ with datum, map projection                             and vary with map projection parameters
An infinite number of possible x,y values for any location

- Typically based on geographic coordinates
*Linked to a specific reference ellipsoid*
*Tied to a specific horizontal datum*
*Usually given as latitude and longitude*
- End up with projected coordinates
*Ultimately based on geographic coordinates*
*X,Y values will differ with datum, map projection                             and vary with map projection parameters*
*An *_*infinite number*_* of possible x,y values for any location*

- Projecting coordinates

**Page  27**

## Projecting coordinates

|  | Map projection selection* | Datum selection |
| --- | --- | --- |
| Large area/small scale 
(~ 1:10,000,000) | Very Important | Not as critical |
| Small area/large scale
(~ 1:24,000) | Not as critical | Very important |

- * Map projection parameters are always important

*Classic cartography*

- _Static geospatial data display_
- Paper map or chart = _*Cartographer*_ selects data and scale.
- Meet National Map Accuracy Standards* (slide 37)*

**Page  28**

## Projecting coordinates

|  | Map projection selection* | Datum selection |
| --- | --- | --- |
| Scale Continuum
Data viewed at any scale
        small               large
              | 
Very Important | 
Very important |

- * Map projection parameters are always important

*Digital cartography*

- _Dynamic geospatial data display_
- Computer map display = _*User*_ selects data and scale.
- Metadata critical, need to state location accuracy.

*(zoom out)*

*(zoom in)*

**Page  29**

## Projecting coordinates

**Unprojected lat-long**
**Lambert Conformal Conic**
**Mercator**

- Three projections
- centered at
- 39° N, 96 ° W

**Page  30**

## Projecting coordinates

- Same dataset    Same datum      Same map projection

*Different CMER*

**Page  31**

- Alber’s Equal Area
- CMER = 95.5o W

- Alber’s Equal Area
- CMER = 80o W

- Alber’s Equal Area
- CMER = 110o W

## Projecting coordinates

**Reference Ellipsoid**

**GRS 80**

**Page  33**

## Projecting coordinates

- From HEC-GeoRAS user manual

**Page  32**

***Infinite number of possible combinations***

## UTM coordinates

- Lat =  40° 00’ 00” N,   Long 90° 00’ 00” W
*Datum = NAD83*
- NORTHING* (Y)*     	EASTING*(X) *            ZONE
- 4,432,069.057 m 	756,099.648 m  	15
- 4,432,069.057  m	243,900.352  m 	16

- The zone makes a huge difference.
- Be careful about precision.

**Page  34**

## UTM coordinates

- Lat =  40° 07’ 00.000” N
- Long  87° 32’ 00.000” W

*          **Datum = NAD83*
- Lat =  40° 07’ 00.133” N
- Long  87° 32’ 00.063” W

-  = 4 m

**Page  35**

## UTM coordinates

**Page  36**

## Map accuracy standards

- National Map Accuracy Standards (circa 1940s)
- Horizontal accuracy
- Map with scale larger than 1:20,000 (such as 1:5,000)
- Not more than 10% of “well defined” points in error by more than 1/30 inch (0.8 mm)
- Maps with scale 1:20,000 or smaller (such as 1:100,000)
- Not more than 10% of “well defined” points in error by more than 1/50 inch (0.5 mm)
- NWS GIS layers derived from 1:2,000,000 scale maps
- 		0.5 mm  2,000,000 = ± 1 km positional accuracy

**Page  37**

## NWS GIS data and NMAS

- ~1 km uncertainty associated with NWS GIS layers derived from 1:2,000,000 scale maps

**Page  38**

## Position uncertainty

**Page  39**

## Position uncertainty

**X**

- Observation data
- uncertainty

- GPS Unit,
- 95% confidence level

- Scaled from a 1:2,00,000 map,     95% confidence level

**X**

**Page  39a**

- 3.2.1 Spatial Accuracy
- Accuracy is reported in ground distances at the 95% confidence level.
- Accuracy reported at the 95% confidence level means that 95% of the positions in  the dataset will have an error with respect to true ground position that is equal to or smaller than the reported accuracy value. The reported accuracy value reflects all uncertainties, including those introduced by geodetic control coordinates,
- compilation, and final computation of ground coordinate values in the product.

## Position uncertainty

**X**

- Observation data
- uncertainty

- GPS Unit,
- 95% confidence level

- Scaled from a 1:2,00,000 map,     95% confidence level

**X**

**Page  39b**

## Position uncertainty and grids

- Ideal grid distance = 10 km

*Actual grid distance = 8  to 12 km*

- Gridded data
- positional accuracy at 95% conf. level
- = ± 10% grid distance

***NWS GIS data***
***Horizontal_Positional_Accuracy_Report: ***
***The data was created from USGS 1:2,000,000 DLG county data and assumed to have the same horizontal accuracy of that data set.***
***(Note - This is ~ 1 km positional accuracy.)***

**Page  40**

## Position uncertainty and grids

- Re-scaled at 10x higher resolution, _but keeping same position accuracy_

- Re-scaled at 10x higher resolution,     _with 10x greater position accuracy_

- Ideal grid distance = 1 km
*Actual grid distance = ????*

- Ideal grid distance = 1 km
*Actual grid distance = 0.8 to 1.2 km*

**Page  41**

## Heights - Ellipsoid and Geoid

- H = Orthometric height (*elevation*)
- h = Ellipsoidal height (*usual GPS output*)
- N = Geoid height (*varies by location*)

**GRS 80 Ellipsoid**

**Topographic surface**

- Geoid height

**Page  42**

## Geospatial data - vertical datums

*a.k.a. Mean Sea Level 1929 (not a tidal datum)*
*Based on 26 coastal tide stations*
- North American Vertical Datum 1988 (NAVD88)
*Based on a gravity station in Canada (Father Pt)*
*Puerto Rico Vertical Datum of 2002 , PRVD02*
*International Great Lakes Datum (IGLD85 tied to NAVD88)*
_*Tied into NAD83, GRS80, and GPS*_
- Additional Tidal and River Datums
*MLLW, MLW, MHW, LAT, CRD,...*

**Page  43**

## Geospatial data - vertical datums

- It’s like having different MSLs for a location.
- Which do you use?
- What problems can occur?

**Page  44**

## Example - elevation

**Page  45**

## Example - elevation

**http://www.progress-energy.com/aboutenergy/powerplants/hydro/**

**Page  46**

## Example - elevation

**15,0****00 ac-ft uncertainty,**
**~10% range**

**Page  47**

- + 1 ft

- - 1 ft

## Example - elevation

**± 2****0,000 cfs uncertainty, ~15% range**

**Page  48**

## Example - elevation

**± 2****0,000 cfs uncertainty, ~15% range**

- Unless otherwise noted, all elevations are in NAVD 88 datum.
- NAVD 88 datum is 0.9 feet lower than NGVD 29 datum.
- Downstream USGS Gage datum referenced to _NGVD29_

**Page  48a**

## Elevation documentation

**Specify either NAVD88 or NGVD29**

- Current NWS products

**Page  49**

## Elevation documentation

**Page  50**

## WSR-88D accuracy study

**Figure 2. **

**Page  51**

## WSR-88D accuracy study

*Least squares fitted line*

**(12.8 km)**

**(161 km)**

**12 km**

**(6.4 km)**

**Page  52**

**Figure 3.**

## WSR-88D accuracy study

| Distance from radar miles (km) | Number of radar data points | Mean Error miles
(km) | % of radar signatures with error of 0.5 miles or greater (0.8 km) | % of radar signatures with error of 1.0 mile or greater (1.6 km) |
| --- | --- | --- | --- | --- |
| 0-20 (0-32) | 52 | 0.2 (0.3) | 13% | 6% |
| 20-50 (80) | 128 | 0.6 (1.0) | 51% | 22% |
| 50-80 (128) | 120 | 0.8 (1.3) | 63% | 33% |
| 80-110 (176) | 105 | 1.1 (1.8) | 79% | 49% |
| 110-140 (224) | 141 | 1.5 (2.4) | 79% | 67% |
| Total | 546 | 0.9 (1.4) | 63% | 39% |

- Table 1. Error distance between tornado location and radar-indicated circulation center as a function of distance from radar.

**Page  53**

## Review

- Define your accuracy needs _before_ a project
- 		*Rule of thumb –10x more accurate ground control *
- Higher resolution grids required greater accuracy data
- 	*	**Don’t confuse accuracy with precision *
- Be aware of data accuracy, scale, datum, projections
- 		*Watch the metadata*
- Never mix datums or projections

**Page  54**

## Hydrology assessment

- Accuracy needs: Using ~4 km HRAP grid
- 	Rule of thumb geospatial control of 10x more accurate
- 		Allowed combined error sources	± 0.4 km
- 	NWS GIS data layers (90% of pts):  ± 1.     km
- 	WSR-88D location (mean diff):         ± 0.1   km (est)
- 	WSR-88D radar derived position:     ± 1.4   km (est)
- 	Estimated cumulative error	         1.7  km
- 	Highest resolution grid ?			  17 km

**Page  55**

- vs 0.4 km

## Hydrologic models

**Page  56**

**Low hanging fruit**
- 1. geospatial issues
- ...

**Medium hanging fruit**
- 1. gridded data input, output
- 2. multi-sensor precip est.
- ....

**High hanging fruit**
- 1. precip micro physics
- 2. unsaturated flow
- ....

**Modeling issues**

## Resources

- http://www.ngs.noaa.gov

**Page  57**