The North Central Texas Council of Governments has facilitated the cooperative purchase of 4-band, 6-inch resolution digital aerial photography (orthos) for over 6,800 square miles of the NCTCOG region, including Collin, Dallas, Denton, Tarrant and Rockwall counties and portions of Kaufman, Johnson, Ellis, Hood, Parker and Wise counties. The orthos were collected in January and February 2011 and are available for purchase via www.dfwmaps.com according to the following price schedule.
|1-foot Digital Orthophotography
||$8.00 per ortho tile
[$37.17 per square mile]
|2-foot Digital Orthophotography
||$5.00 per ortho tile
[$23.23 per square mile]
|4-foot Digital Orthophotography
||$3.00 per ortho tile
[$13.94 per square mile]
|8-foot Digital Orthophotography
||$2.00 per ortho tile
[$9.29 per square mile]
All images are georeferenced TIFF files and are sold as whole tiles only; price per square mile is provided only as a reference. Each TIFF ortho tile is 3,000' x 2,000', or 0.215 square miles. Sample images and a shapefile of the tile structure may be downloaded below; all orders for 2011 orthophotography products can be placed via www.dfwmaps.com. Before placing an order, read the frequently asked questions.
1-Foot Resolution Color Ortho Sample [TIFF format with world file; 14 MB]
TIFF orthophotography grid [ArcView Shapefile]
The specifications have been provided by the contractor for this project, Sanborn.
Color aerial photography is acquired using a Z/I Digital Mapping Camera (Z/I DMC) which has been proven accurate for photogrammetry mapping by the USGS digital sensor evaluation program at Stennis, Mississippi. The flight design achieved a nominal ground-pixel resolution sufficient for developing 6" pixel orthophotography without oversampling. It was also designed with sufficient forward overlap and strip side laps to ensure total project area coverage. The following flight specifications were used for aerials and contours:
|Altitude of Capture
|| Approx. 5,000'-8,500' above mean terrain
|| 120 mm (4.72")
|| Minimum: 1"= 1764' (Photo scale 1:21167)
Maximum: 1" = 2129' (Photo scale 1:25552) – Main block DFW restricted airspace
||January 21 - February 28, 2011
|| Leaf off, cloud free, 30 degree minimum sun angle
||60% (80% for True Ortho areas)
||30% (60% for True Ortho areas)
||Texas State Plane, North Central Zone
||US Survey Feet
The coverage area for 2011 includes Dallas, Fort Worth, and surrounding urban and rural counties. Two areas of true orthoimagery were produced over the downtown areas of Dallas and Fort Worth.
Control is collected in Texas State Plane, North Central Zone, NAD 83, NAVD 88 (HARN), U.S. Survey Feet. Control is established using Dallas and Fort Worth Virtual Reference Stations (VRS) and control is targeted prior to flight to ensure visibility of the location in the imagery.
Aerotriangulation is performed using industry-accepted procedures on an approved softcopy workstation or analytical stereoplotter to support the horizontal accuracy requirements of the digital orthophotography. The RMS of known ground points will not exceed 1 foot in x, y, or z.
The imagery were produced using best-available DEM data, including DEM data provided by NCTCOG and LiDAR acquired over Dallas County and portions of surrounding counties in 2010. The orthoimagery were colorbalanced and to radiometric parameters established during the pilot/prototype phase. The most nadir part of every image was used in the mosaicking. Mosaic seam lines were created to ensure that joins do not cut hard detail where avoidable. Mosaic lines do not cross through above-ground structures unless unavoidable. Seams through or along streets are placed to avoid obscuring or artificially creating centerlines, curbs, and sidewalks. Quality control steps were performed during production of the orthoimagery to ensure the orthoimagery is inspected for compliance with required aesthetic and spatial criteria.
The final orthoimagery meet National Map Accuracy Standards (NMAS) 1"=100' scale mapping (RMSE 1.55' x or y). NCTCOG's review team inspected the orthoimagery using Sanborn's GeoServe™ web-based quality assurance system, which allows reviewers to immediately communicate issues found in the imagery through placement of attributed points. Any non-conforming orthoimagery was corrected and posted to the application for approval by the review team. The final orthoimagery were delivered tiled to the NCTCOG grid (2,000' x 3,000' tiles), in 4-band (RGB+NIR) TIFF format and 3-band (RGB) MrSID compressed format (Generation 2, 18:1 compression ratio) tiled to the NCTCOG MrSID grid (18,000' x 10,000' tile).
For questions about the 2011 digital aerial photography project, contact Shelley Stenoien at 817-695-9156 or via email.