Evaluation of TXDOT’s Digital Base Map By Michael Flaming
Jan 12, 2016
Evaluation of TXDOT’s Digital Base Map
ByMichael 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
Trimble Pro XRS Receiver
Trimble Recon Data Collector
GPS Data Collection
1989 Nissan Maxima w/sunroof
GPS Data
US183
I 35
LP 360
TXDOT Base Map
GPS Data
Focus area LP360
GPS (Maxima) Line
TXDOT Base map Line
Calculate buffer
10m buffer of GPS line
Calculate Intersect
Intersect of 10m buffer of GPS line and
TXDOT Basemap
3m buffer of GPS line
Intersect of 3m buffer
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
Attribute table
Omitted data Portions that were obviously outdated due
to recent construction were omitted.
LP 360 GraphBasemap Accuracy
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
0 1 2 3 4 5 6 7 8 9 10
Buffer (m)
Co
vera
ge
Rat
io
LP360
Basemap Accuracy
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
m 1 m 2 m 3 m 4 m 5 m 6 m 7 m 8 m 9 m 10 m
Buffer (m)
Cov
erag
e R
atio
LP1
LP360
I-35
US183 Urban
2222
FM672
US183 Rural
Mean
Upper Bound
Lower Bound
Coverage Ratio
Conclusions
Linear features of TXDOT roadbed basemap have an accuracy of +/- 7m with a confidence of 90%.
Questions?