accuracy assessment of nextmap elevation data for the state of alabama m. lorraine tighe phd...
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Accuracy Assessment of NEXTMap Elevation Data for the State of AlabamaM. Lorraine Tighe PhD Candidate
Director, Geospatial Solutions - Intermap
November 18, 2011
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Agenda
IntroductionObjectivesStudy AreaDataAnalysisResultsDiscussionConclusions
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Introduction
This paper presents the results of the vertical accuracy of NEXTMap derived bare ground elevation data over a variety of land cover types by comparing it against National Geodetic Survey (NGS) reference points and against the United States Geological Survey National Elevation Data (NED) for the State of Alabama.
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Objectives
Investigate the accuracy of the NEXTMap data for the State of Alabama against survey control reference data (1126 available NGS) and 10 m NED data.
Investigate barren and obstructed land cover of all slopes.
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Study Area
State of Alabama
Area: 52,419 mi2 (135,765 km2)
Elevation Range: sea level to 2,413 ft (735.5 m)
Water Land Cover: 3.19% (1,672 km2)
Forest Land Cover: 67% 22 million ha (89,000 km2)
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Data
Elevation Data:• 5 m NEXTMap
DTM• 10 m NED DTM
Reference Data:• 1126 National
Geodetic Survey (NGS) geodetic data points
1126 NGS control points
7 © 2011 Intermap Technologies. All rights reserved.
Collection – to – End Product: NEXTMap
Collection Production Aggregation Applications Distribution
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The Core Technology - IFSAR
Note: STAR-3i uses GPS ground station for differential processing; no other ground control points are required.
Interferometric Synthetic Aperture Radar - IFSAR
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IFSAR Processor to Derive Height Measurements
The variation in phase difference from pixel to pixel can be converted into relative change in surface elevation through a set of (closed form) equations to yield x, y, and z.
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z(y)
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Digital Surface Model (DSM)
DSM: Winston County, Alabama. Confluence of Right Fork Clear Creek and Clear Creek. Also showing US Highway 278.
DSM: Winston County, Alabama. Confluence of Right Fork Clear Creek and Clear Creek. Also showing US Highway 278.
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Digital Terrain Model (DTM)
DTM: Winston County, Alabama. Confluence of Right Fork Clear Creek and Clear Creek. Also showing US Highway 278.
DTM: Winston County, Alabama. Confluence of Right Fork Clear Creek and Clear Creek. Also showing US Highway 278.
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Handling Obstructed and Void Areas
The acquisition plan is designed to minimize the percent of void data in the output product.
We have a fully integrated terrain solution (FITS) that can utilize modified DSM data or ancillary data to fill in voids and to help recalculate the terrain surface beneath vegetation canopies.
e.g. Before the “Void Infill” process has been automatically run, void areas have an interpolated appearance
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Terrain Solution
We use a modified version of our multiple DSM passes or ancillary data to rebuild the DTM in areas of void and obstruction.
We will use available ancillary data as a last resort.
e.g. After the “Void Infill” process has been automatically run, the ancillary DEM has a more natural appearance
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Analysis
Visual assessments, in particular, drainage features were conducted.
Statistical analysis (RMSE, mean error, standard deviation, cumulative frequency plot) is implemented with the use of the NGS geodetic data points to calculate the vertical accuracy of the NEXTMap and NED data.
Data Characterizations: land cover analysis to characterize where to expect the range of vertical accuracies.
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Visual Assessment Results
Intermap’s NEXTMap data for the State of Alabama is current, as a majority of the state was collected with the same sensor between 2005 and 2008. By comparison, a majority of the vintage NED that is available for the state was collected between 1960 and 1979. Portions of the NED were collected prior to 1959.
Different vintages and resolutions result in different drainage delineations.
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Visual Assessment Results
NEXTMap
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Statistical Results (1)
Land Cover Slope Statistic (m) NEXTMap NED
Barren <10o
Mean error -0.19 0.13StDev 0.51 1.35RMSE 0.55 1.36
Vegetation <10o
Mean error -0.5 -0.27StDev 1.38 1.53RMSE 1.47 1.56
Dense Vegetation 0o - 29o
Mean error -0.74 -0.5StDev 2.59 3.66RMSE 2.99 3.67
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Statistical Results (2)
Cumulative percent error plot (Barren, low slopes)to help to visualize what percentage of data can be expected to meet various accuracies.
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DEMs vs. VCP
NED SRTM_DT2 DTM DSM
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Alabama Data Characterization
A snapshot of the data characterization of the State of Alabama, with respect to the accuracy to be expected by NEXTMap data.
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Conclusions
Vertical Accuracy Assessment (expressed in RMSE): • 0.55 m in barren terrain • 1.27 – 3 m in obstructed terrain (e.g. vegetation/urban)
Hydrological analysis: • NEXTMap more accurate representation of water ways
(e.g. lakes, ponds, rivers, streams) than NED• difference in DTM resolution: 5m NEXTMap, 10 m NED• temporal differences: NEXTMap 2008; NED ~ 20 years
