NWS Geospatial Accuracy Issues

NWS Geospatial Accuracy Issues

Rick Koehler, PhD

NOAA – NWS – OCWWS – FDTB

Boulder, CO

NFUSE presentation

April 9, 2008

[email protected]

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

ACUF White Paper


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.

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

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


“2 miles ahead” vs

“2 miles, ± 0.1 miles”or

“2 miles, ± 2.0 miles”

“Comparison”


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


Accuracy measuresBut what about positional error?


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


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.


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


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)


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


Position uncertainty - polygons

Polygon may be drawn larger if key locations show a larger 2drms circle

Polygon “landmarks”


RIDGE geospatial errorsPosition accuracy check

Get readout of a known pointMD – DE – PA tri-state point “MDP Corner” survey mark


RIDGE geospatial errors

PA

MD

DE

FireFox4.1 km (2.5 mi) displacement IE 7

3.9 km (2.4 mi) displacement2.6 km


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


Geospatial experiment


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


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


Estimated accuracy of NWS data

NMAS = Natl Map Accuracy Standards

Data sources and comparisons


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?


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.


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?

NWS Geospatial Accuracy Issues

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