Evaluation of TXDOTs Digital Base Map

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Evaluation of TXDOTs Digital Base Map

• By
• Michael Flaming

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Digital Map Standards

• National Map Accuracy Standard
• 90 of tested points smaller that threshold
• 124,000 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

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Aerial Photo Metadata

• Map Name  CAPCO 2002 Orthophotography Project
  Imagery
• Photo Scale           13000'
• 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

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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

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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

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GPS Data Collection
Trimble Pro XRS Receiver
Trimble Recon Data Collector
1989 Nissan Maxima w/sunroof
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GPS Data
I 35
LP 360
US183
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TXDOT Base Map
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GPS Data
Focus area LP360
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GPS (Maxima) Line
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TXDOT Base map Line
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Calculate buffer
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10m buffer of GPS line
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Calculate Intersect
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Intersect of 10m buffer of GPS line and TXDOT
Basemap
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3m buffer of GPS line
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Intersect of 3m buffer
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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

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VBA Script to Calculate Length

• Gathers up all the bits of lines and adds up
  the sum
• Separated by object ID

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New Line length

• Field calculation to get new line length

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Resulting data for LP 360

• Nine attribute tables for each buffer intersect
  dumped to Excel for analysis

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Attribute table
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Omitted data

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

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LP 360 Graph
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Coverage Ratio
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Conclusions

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

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Questions?