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

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0.90

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0 1 2 3 4 5 6 7 8 9 10

Buffer (m)

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vera

ge

Rat

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LP360

Basemap Accuracy

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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?