NCTCOG has obtained 2-foot digital elevation contours for all 16 counties in the NCTCOG region as part of the 2007 aerial photography cooperative purchase. The contractor for this project was Bohannan Huston.
Members of NCTCOG and private entities may purchase digital elevation contours at a cost of $10 per tile. Contours are sold as whole tiles only. Each tile is 3,000' x 2,000', or 0.215 square miles. Available file formats are ArcView shapefile (.shp) and AutoCad (.dxf).
Sample files and a shapefile of the tile structure may be downloaded below; all orders for digital elevation contour products can be placed via www.dfwmaps.com. Before placing an order, read the instructions and frequently asked questions.
Stereo images were processed to automatically generated a digital elevation model. Any non-man made structures and vegetation were removed from the digital surface model. MicroStation design files containing ground elevations were clipped to eliminate overlap between stereomodels and filtered to declassify points that constituted excessively small elevation spikes.
Surface Type
Auto-correlated surface
Altitude of Capture
No higher than 5,500' above mean terrain
Capture Period
January – March 2007
Control Sources
Control panels were laid down and their positions recorded with static GPS. Positions were then corrected and network adjusted.
DEM Point Spacing
8 feet
DEM Point Accuracy
1.5 feet (horizontal)
Contour Interval
2 feet
Coordinate System
Texas State Plane, North Central Zone
Horizontal Datum
NAD 83
Vertical Datum
NAVD 88
Units
US Survey Feet
DEM Format
ASCII
Contour Format
Shapefile
Source stereo images are collected at an elevation of no more than 5500 feet above ground. Theoretical design of the photogrammetric flight indicate a vertical accuracy will be achieved for an ASPRS vertical supporting 2.0' contour intervals. Contours have been generallized and smoothed which may slightly affect attributed elevation accuracy for the positional sample.
Aerotriangulation results for all raw imagery was checked for horizontal accuracy. Digital auto-correlated elevations correspond to the accuracy of the aerotriangulation. NSSDA validation indicates a vertical RMSE of .54' yeilding a 95% confidence of 1.06 feet vertical on the digital surface model from where the contours were generated. Subsequent contour generalizing and smoothing will affect the overall positional accuracy of the contours.
The North Central Texas Council of Governments (NCTCOG) coordinated the capture of a photogrammetric surface using auto-correlation photogrammetric analysis techniques. The contour data are intended to aid in engineering planning. They are not intended to furnish measurements accurate enough for engineering design work. The NCTCOG provides these geographic data "as is" and makes no guarantee or warranty concerning the accuracy of information contained in the geographic data. NCTCOG further makes no warranties, either expressed or implied, as to any other matter whatsoever, including, without limitation, the condition of the product, or its fitness for any particular purpose. The burden for determining fitness for use lies entirely with the user.
Although these data have been processed successfully on computers of Bohannan-Huston, Inc. (BHI) and served by NCTCOG, no warranty, expressed or implied, is made by NCTCOG or BHI regarding the use of these data on any other system, nor does the fact of distribution constitute or imply any such warranty. In no event shall NCTCOG or BHI have any liability whatsoever for payment of any consequential, incidental, indirect, special, or tort damages of any kind, including, but not limited to, any loss of profits arising out of use of or reliance on the geographic data or arising out of the delivery, installation, operation, or support by NCTCOG.
Known issues with the 2007 surface and contour datasets:
Where there are large areas of dense ground vegetation or tree canopy the density of verifiable ground control decreases which in turn decreases the accuracy of the surface and contour data in those areas. These areas can potentially be identified by long straight or more generalized contours.
To create a cartographically pleasing contour the lines have been smoothed slightly. This smoothing has a small impact on accuracy but creates a much easier to use contour dataset.
For questions about purchasing digital elevation contours, contact Shelley Broyles at 817-695-9156 or via email.
North Central Texas Council of Governments | 616 Six Flags Drive P.O. Box 5888 Arlington, TX 76005-5888
Main Operator: (817) 640-3300 | Fax: (817) 640-7806
