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Evaluation of TXDOT’s Digital Base Map. By Michael Flaming. Digital Map Standards . National Map Accuracy Standard 90% of tested points smaller that threshold 1:24,000 threshold for printed maps is 1/30 th of an inch = 66.66 feet No national standard for digital maps
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Evaluation of TXDOT’s Digital Base Map By Michael Flaming
Digital Map Standards • National Map Accuracy Standard • 90% of tested points smaller that threshold • 1:24,000 threshold for printed maps is 1/30th of an inch = 66.66 feet • No national standard for digital maps • some claim to conform to NMAS
Aerial Photo Metadata • Map Name: CAPCO 2002 Orthophotography Project Imagery • Photo Scale: 1=3000' • Horizontal Datum: NAD83 • Coordinate system: US State Plane • State Plane Zone: Texas Central • State Plane FIPSZone: 4203 • Projection: Lambert Conformal Conic • Photo Flight Date: February 2002 • Camera Name: RC30 5236 • Film Type: Kodak Aerocolor III 2444 • Focal Length: 153.026mm • Aircraft Height (AMT): 18000' • Horiz. Accuracy Std. +/- 4' • Bounding Box in US State Plane Zone 4203 • Upper Left: 3231310, 10022634 • Upper Right: 3241957, 10022634 • Lower Left: 3231310, 10010542 • Lower Right: 3241957, 10010542 • Description: Color Aerial Photography captured for the • 10,496 square mile region database as developed and • maintained by the Capital Area Planning Council (CAPCO). • Distributed by CAPCO 2512 IH 35 South, Suite 220 Austin, TX 78704
Evaluate Horizontal Accuracy • Compare Digital maps against more accurate standard • Accuracy of digital maps on the order of 5-10m • Accuracy of differentially corrected GPS on the order of < 0.5m
Analysis of Accuracy • Test routes selected • Urban Interstate Highway (I-35) • Urban State Highway (US 183, Mopac) • Rural State Highway (US 183 Lockhart) • Loop 360, 2222, FM163 • High accuracy GPS data collected • Actual road data compared to same road segments of digital maps • Various buffers used to compute map coverage ratio • Buffer represents range in which results are accurate • For a given coverage ratio, the larger the buffer the lower the accuracy • For a given buffer, the greater the coverage ratio, the higher the accuracy
GPS Data Collection Trimble Pro XRS Receiver Trimble Recon Data Collector 1989 Nissan Maxima w/sunroof
GPS Data I 35 LP 360 US183
GPS Data Focus area LP360
TXDOT Base map Line
Attributes of Intersect • Same as attributes of base map • Shape length in decimal degrees • Original Projected line length in meters • Must calculate new shape lengths
VBA Script to Calculate Length • “Gathers” up all the bits of lines and adds up the sum • Separated by object ID
New Line length • Field calculation to get new line length
Resulting data for LP 360 • Nine attribute tables for each buffer intersect “dumped” to Excel for analysis
Omitted data • Portions that were obviously outdated due to recent construction were omitted.
Conclusions • Linear features of TXDOT roadbed basemap have an accuracy of +/- 7m with a confidence of 90%.