Evaluation of TXDOT’s Digital Base Map

1. Evaluation of TXDOT’s Digital Base Map By Michael Flaming

2. 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

3. 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

4. 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

5. 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

6. GPS Data Collection Trimble Pro XRS Receiver Trimble Recon Data Collector 1989 Nissan Maxima w/sunroof

7. GPS Data I 35 LP 360 US183

8. TXDOT Base Map

9. GPS Data Focus area LP360

10. GPS (Maxima) Line

11. TXDOT Base map Line

12. Calculate buffer

13. 10m buffer of GPS line

14. Calculate Intersect

15. Intersect of 10m buffer of GPS line and TXDOT Basemap

16. 3m buffer of GPS line

17. Intersect of 3m buffer

18. 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

19. VBA Script to Calculate Length • “Gathers” up all the bits of lines and adds up the sum • Separated by object ID

20. New Line length • Field calculation to get new line length

21. Resulting data for LP 360 • Nine attribute tables for each buffer intersect “dumped” to Excel for analysis

22. Attribute table

23. Omitted data • Portions that were obviously outdated due to recent construction were omitted.

24. LP 360 Graph

25. Coverage Ratio

26. Conclusions • Linear features of TXDOT roadbed basemap have an accuracy of +/- 7m with a confidence of 90%.

27. Questions?