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Digital Orthophotography Frequently Asked Questions

What is the difference between an aerial photograph and a digital orthophotograph?

A conventional perspective aerial photograph contains image displacements caused by the tilting of the camera and terrain relief (topography). It does not have a uniform scale. You cannot measure distances on an aerial photograph like you can on a map because it is not a map.

An orthophoto is a uniform-scale photograph, in which effects of tilt and relief are removed from the aerial photograph by the rectification process. An orthophoto is, essentially, a photographic map. Since it has a uniform scale, it is possible to measure directly on it like other maps, and therefore it may serve as a base map onto which other map information can be overlaid.

For more information, see online.wr.usgs.gov/ngpo/doq/doq_basics.html.

What is the difference between frame-based and pushbroom orthophotography?

The major two technologies for aerial large format digital imaging are the frame imaging approach as implemented in the Microsoft-UltraCam and the Intergraph-DMC and the linear array technology by Leica in their ADS-40 and up. The large format framing cameras “stitch“ an image from individually collected smaller image segments or tiles with a sophisticated technology to ensure a geometrically accurate and seamless single large format image.

Frame-based orthophotography captures imagery on film. High-precision manufacturing of camera elements such as lens, body, and focal plane; rigorous camera calibration techniques; and continuous improvements in electronic controls have resulted in a mature technology capable of producing stable, geometrically well defined, high accuracy image products. Lens distortion can be measured precisely and modeled, and image motion compensation mechanisms remove the blur caused by aircraft motion during exposure. Aerial film is developed using chemical processes and then scanned at a high resolution. In today's photogrammetric production environment, virtually all aerotriangulation, elevation, and feature extraction are performed in an all-digital work flow.

Pushbroom orthophotography produces one large file per linear array and flight line. The image “strip” is therefore a collection of perhaps 6 or 7 files, one for each of 4 color channels, plus 2 or 3 panchromatic strips for stereo work. The strip image concept is not compatible with photogrammetric tradition. The image itself does not encode any geometric information, but is entirely defined by the motion of the platform. The central perspective is only applicable in the cross track direction, whereas the along track direction is an orthogonal projection. Therefore a separate workflow is needed for photogrammetric processing of data from “pushbrooming” sensors.

Either system can be used to acquire highly accurate orthophotography.

For more information about optical sensors, see www.e-education.psu.edu/geog480/node/444. For more information about frame versus pushbroom aerial cameras, see pure.tugraz.at/portal/files/1450315/289A - UltraCam Microsoft-TechReport on FAQ, 2007.pdf.

How do I place an order?

Orthophotography can be purchased online at http://www.dfwmaps.com/purchase.

What is a tile and how big are they?

In order to manage the data efficiently, the data from the flyover are separated into tiles. The tiles are imported into mapping software and mosaicked (pieced) together so they appear as one seamless image on DFWmaps.com. Each tile is 3,000 feet wide by 2,000 feet high.

What is the graphic format of the aerial photography? 

TIFF (Tagged Image File Format). If you receive compressed imagery, it will be in MrSID (Multi-resolution Seamless Image Database) format. MrSID imagery cannot be ordered throughDFWmaps.com.

How is the order delivered?

Unless your order is very large, it will be delivered via FTP. If you wish to discuss a different method of delivery for your order, please contact us at gisorders@nctcog.org.

After you submit an order, you will receive an email containing the link to the ftp location containing your data. The process does not happen immediately. You will typically get an email for data download within 2-3 hours of placing an order.

If you request another delivery method, such as pickup or Express Mail, NCTCOG staff will contact you to determine the preferred media for delivery, and the estimated time to completion.

Note that the time for pickup or Express Mail could be longer than FTP, because special arrangements must be made to copy the data to media and arrange delivery.

How long will it take to process my order?

The average order takes 2-3 hours to process. If you request a delivery method other than FTP, it will take longer.

What is a .tiff?

A .tiff is a graphic image, much like a .bmp or .jpeg. The images are orthophotographs of the NCTCOG region and have assigned geographic coordinates. Rather than lay on top of one another, TIFFs lay side by side.

What is resolution?

Resolution is the size of each pixel in the graphic file. Much like a television screen, orthophotography is a collection of small boxes, or pixels, each having an assigned color. Together the pixels make up an orthophoto. The smaller the pixel the more precise (clear) the orthophoto, however the file size and cost increase with the resolution.

What resolutions are available and how much do they cost? 

Orthophotography costs are based on the year flown and the resolution. The following chart summarizes the cost per tile for the years and resolutions available.

Year 3 inch 6 inch 1 foot 2 foot 4 foot 8 foot
2016 -   $36.00  $18.00  $9.00  $5.00  $3.00 
2015 -   $36.00  $18.00  $9.00  $5.00  $3.00 
2013 -   $24.00  $12.00  $6.00  $3.00  $2.00 
2012 -   $18.00  $9.00  $6.00  $3.00  $2.00 
2011 -   -   $8.00  $5.00  $3.00  $2.00 
2009 -   $12.00  $6.00  $4.00  $2.00  $1.00 
2007 -   $10.00  $5.00  $3.00  $2.00  $1.00 
2005 -   $10.00  $5.00  $3.00  $2.00  $1.00 
2003 -   -   -   $3.00  $2.00  $1.00 
2001 -   -   -   $3.00  $2.00  $1.00 

 

What is the file size for a tile? 

3 inch = 280 MB 
6 inch = 71.5 MB 
1 foot = 17.9 MB 
2 foot = 4.5 MB 
4 foot = 1.1 MB 
8 foot = 280 KB

Where can I view or download samples? 

Samples of aerial photography may be downloaded via the following links.

What counties have aerial photography?

Orthophotography and Elevation Contours Coverage Areas

2017 Orthophotography is available in 6-inch resolution for all 16 counties in the NCTCOG region, plus Jack County. It was also available at 3-inch resolution for select cities.

2015 Orthophotography is available in 6-inch resolution for Collin, Dallas, Denton, Rockwall And Tarrant counties and portinos of Hunt, Kaufman, Ellis, Johnson, Hood, Parker and Wise counties.

2013 Orthophotography is available in 6-inch resolution for Collin, Dallas, Denton, Tarrant and Rockwall counties and portions of Hunt, Kaufman, Ellis, Johnson, Hood,  Parker and Wise counties.

2011 Orthophotography is available in 1-foot resolution for Collin, Dallas, Denton, Rockwall, and Tarrant counties and portions of Ellis, Hood, Kaufman, Johnson, Parker and Wise counties.

2009 Orthophotography is available in 6-inch resolution for Collin, Dallas, Denton, Rockwall, and Tarrant counties and portions of Ellis and Johnson counties.

2007 Orthophotography is available in 6-inch resolution for all 16 counties in the NCTCOG region.

2005 Orthophotography in 6-inch resolution is available for the following eight counties: Collin, Dallas, Denton, Ellis, Kaufman, Navarro, Rockwall and Tarrant.

2003 Orthophotography is available in 1-foot resolution for all 16 counties in the NCTCOG region. Orthos are also available in 6-inch resolution for Collin, Dallas, Denton, Ellis, Kaufman, Navarro, Rockwall, and Tarrant counties.

2001 Orthophotography is available in 1-foot resolution for the following 11 counties: Collin, Dallas, Denton, Ellis, Hunt, Johnson, Kaufman, Navarro, Rockwall, Somervell and Tarrant.

Where can I view the specifications? 

What is the accuracy of the orthophotography? 

The aerial photography meets National Map Accuracy Standards at a map scale of 1 inch = 200 feet (1:2400). The photo scale is 1' = 1500' (1:18000).

Can aerial photography be certified for legal purposes?

An aerial image can be certified as a NCTCOG product only. Certification costs $50 per item and must be done in person at NCTCOG by a notary public. Contact Alice Webster at 817-695-9140 for more information.  To certify the accuracy of the aerial image, contact the company which flew the aerials (see below).

Can TIFF files be used in AutoCAD or Microstation?

Yes, if the software version can read a TIFF file with an associated world (.tfw) file.

Who flew the aerial photography? 

2001 - Vargis, LLC
2003 - Pixxures Inc. and Dallas Aerials Surveys Inc. 
2005 - Bohannan Huston
2007 - Bohannan Huston
2009 - Bohannan Huston
2011 - Sanborn
2013 - Woolpert
2015 - Woolpert
2017 - Woolpert

What time of year is the aerial photography taken? 

Orthophotography is flown in "leaf-off" conditions, which typically occurs January through March.

Are the street names that appear on DFWmaps.com on the orthophotography? 

No. The street names are provided by Microsoft Bing Maps and are not included on NCTCOG orthophotography.


  

 

 
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