Download - NWS Geospatial Accuracy Issues
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NWS Geospatial Accuracy Issues
Rick Koehler, PhD
NOAA – NWS – OCWWS – FDTB
Boulder, CO
NFUSE presentation
April 9, 2008
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April 9, 2008 NFUSE team presentation
Presentation purpose and outlinePossible NFUSE discussion topic - geospatial issues
ACUF White Paper (recommendations 4.0 and 5.0)
Why even worry?
“Comparison” - GPS technology and NWS product
Accuracy measurements
Position uncertainty
RIDGE geospatial errors
Geospatial experiment
Reference - NSRS
Estimated accuracy of NWS data
Needs assessment and standards
Summary
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ACUF White Paper
April 9, 2008 NFUSE team presentation
Recommendation 4.0:
NWS should develop and maintain the ability to produce objective uncertainty
information from the global to the regional scale.
To know the full, usable spatial resolution, it is critical to know the quality and accuracy of the geospatial component.
Geospatial data appropriate at one scale may not be appropriate at a different scale.
Recommendation 5.0:
To ensure widespread use of uncertainty information, NWS should make all raw and
post-processed probabilistic products easily accessible to the Enterprise at full
spatial and temporal resolution. Sufficient computer and communications resources
should be acquired to ensure effective access by external users and NWS personnel.
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Why even worry?
“Weather is so dynamic and models are so imprecise, we don’t have to worry about location.”
Highly variable precipitation field
Important to have exact position
Uniform precipitation field
Not critical to have exact position
Often heard comment within NWS
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Why even worry?
“It is now generally recognized that error, inaccuracy, and imprecision can ‘make or break’ many types of GIS projects.
That is, errors left unchecked can make the results of a GIS analysis almost worthless.”
Error, Accuracy, and Precision
The Geographer's Craft Project - 1995
Ken Foote and Donald Huebner Department of GeographyThe University of Colorado at Boulder
http://www.colorado.edu/geography/gcraft/notes/error/error_f.html
For NWS, let AWIPS = GIS
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April 9, 2008 NFUSE team presentation
“2 miles ahead” vs
“2 miles, ± 0.1 miles”or
“2 miles, ± 2.0 miles”
“Comparison”
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April 9, 2008 NFUSE team presentation
Accuracy measures
1. Attribute accuracy
2. Position accuracy
3. Lineage
4. Logical consistency
5. Completeness
MacEachren et al.Visualizing Geospatial Information Uncertainty: What We Know and What We Need to KnowCartography and Geographic Information Science, Vol. 32, No. 3, 2005, pp. 139-160
Data quality categories for Spatial Data Transfer Standards (SDTS)
15
10 13
9
11
5
Attribute
Position
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April 9, 2008 NFUSE team presentation
Accuracy measuresBut what about positional error?
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April 9, 2008 NFUSE team presentation
Lat/Long listed for this point.
But is the object really at this exact point?
Accuracy measures
Circle error probability (CEP) - 50% chance true position within this circle
Diameter root mean square ∙ 2 (2drms) - 95% prob used by NOAA/NOS
CEP
2 drms = 2.4 CEP2 drms
Uncertainty as a distance
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April 9, 2008 NFUSE team presentation
X
Position data is probabilistic, not deterministic
GPS Unit, 95% confidence level
Scaled from a 1:2,00,000 map,
90% confidence level
Position uncertainty
X
15 meters at 95% conf. level = 707 m2 circle (2.7 x 10-4 mi2)
1,000 meters at 90% conf. level = 3.14 km2 circle (1.2 mi2 or 768 acres)
NOS: Total error in position will not exceed 5 meters at the 95 percent confidence level.
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April 9, 2008 NFUSE team presentation
Position uncertainty – “field data”
2drms not stated
2drms = 0.10 map unit
2drms = 0.25
2drrms = 0.50
Station "Observed" X Y X Y X Y X YA 0.587 1 1 1.01 0.91 1.14 1.03 0.97 1.14B 0.413 1 2 1.03 1.91 0.99 2.21 1.23 1.88C 0.750 1 3 0.99 2.95 0.99 3.10 0.90 2.91D 0.038 1 4 1.01 3.97 0.97 3.93 0.81 4.26 E 0.195 2 1 1.97 0.94 1.88 1.09 2.04 1.02F 0.899 2 2 2.02 1.96 1.99 2.19 2.38 1.89G 0.978 2 3 2.04 2.97 1.82 2.81 1.83 2.39H 0.662 2 4 2.00 4.01 1.81 3.96 2.02 4.05 I 0.979 3 1 3.06 0.92 3.05 0.74 3.31 0.73J 0.511 3 2 3.10 1.99 2.83 2.11 3.14 1.82K 0.706 3 3 3.06 2.92 3.16 3.00 2.78 2.95L 0.305 3 4 2.96 4.08 3.02 4.13 3.06 3.70
M 0.265 4 1 4.10 0.94 3.99 0.96 3.57 0.82N 0.319 4 2 3.98 1.96 4.09 2.14 3.98 1.89O 0.001 4 3 4.02 3.07 4.05 3.14 4.05 2.26P 0.046 4 4 3.99 3.95 3.95 4.01 3.92 4.34
High accuracy
(example 1)
Medium accuracy
(example 2)
Low accuracy
(example 3)
Assumed location
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April 9, 2008 NFUSE team presentation
Position uncertainty – contour plot
High accuracy2drms = 0.10
Medium accuracy2drms = 0.25
Low accuracy2drms = 0.50
Assumed location
Actual location
(example 1)
Actual location
(example 2)
Actual location
(example 3)
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April 9, 2008 NFUSE team presentation
Gridded data basis = 1 km x 1 km
Position uncertainty - grids
Possibility of corners at the same location!
Case 1
Position uncertainty 2drms = 1,000 m
Case 2
Position uncertainty 2drms = 100 m
Much more likely corners correctly located
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April 9, 2008 NFUSE team presentation
Position uncertainty - polygons
Polygon may be drawn larger if key locations show a larger 2drms circle
Polygon “landmarks”
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April 9, 2008 NFUSE team presentation
RIDGE geospatial errorsPosition accuracy check
Get readout of a known pointMD – DE – PA tri-state point “MDP Corner” survey mark
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April 9, 2008 NFUSE team presentation
RIDGE geospatial errors
PA
MD
DE
FireFox4.1 km (2.5 mi) displacement IE 7
3.9 km (2.4 mi) displacement2.6 km
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April 9, 2008 NFUSE team presentation
Find the latitude and longitude of the flagpole
Unit has several different “map datums” built into the software
(new ones have 70+ datums)
Geospatial experimentMultiple map datums
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April 9, 2008 NFUSE team presentation
Geospatial experiment
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April 9, 2008 NFUSE team presentation
Topography
Shoreline
Boundaries
Urban zones
Roads
Streams
Vegetation
LongitudeLatitude
Accurate, stable and consistent survey network, datums, map projections, GPS, detailed data -NAD83, NAVD88
Reference - NSRS
Fed govt standardEntrusted to NOAA
National Spatial Reference System
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April 9, 2008 NFUSE team presentation
Reference - NSRSNWS data and geospatial reference frameworks
* Meteorological models may be based upon the International Spheroid of 1924, an reference shape never used for US surveying and mapping
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April 9, 2008 NFUSE team presentation
Estimated accuracy of NWS data
NMAS = Natl Map Accuracy Standards
Data sources and comparisons
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April 9, 2008 NFUSE team presentation
Needs assessmentGeospatial questions
What’s the NFUSE assumed geospatial accuracy?
What are your tolerances?
Unknown data accuracy – how is this handled?
?
Low accuracy, forecast affected
High accuracy, no forecast affects At what point does
geospatial uncertainty begin to influence
forecast uncertainty?
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April 9, 2008 NFUSE team presentation
Position accuracy standards
3.2.1 Spatial Accuracy
Accuracy is reported in ground distances at the 95% confidence level.
This means that 95% of the positions 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.
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April 9, 2008 NFUSE team presentation
Summary
How does geospatial uncertainty affect forecast uncertainty?
Location error can affect some products (contours, polygons)
Inconsistent geospatial data exists with NWS
Established reference framework maintained by NOAA
Existing federal standards for data accuracy
Accuracy needs assessment should be considered
Has this been taken into account?