Digital Elevation Contours FAQ

What is a contour? 

A contour is a set of elevation lines. Elevation is the distance, in feet, above sea level. Lines are positioned every 2 feet in elevation. Flat areas have very few lines spread far apart. Areas with radical shifts in terrain have many lines grouped close together.

What is a tile and how big are they?

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

How were the data captured? 

The 2001 contours were captured using a technique called LiDAR (LIght Detection And Ranging). A laser device in a plane shoots down pulsed laser light which reflects off of any surface it hits. Once the light hits an object, a digital point is laid down. Spacing of these points is approximately 5 meters apart, or about every 16.4 feet. This "grid" of points gives x ,y and z coordinates for everything the laser device touched, including the ground, buildings, trees, etc. This grid is called a Reflective Digital Elevation Model (DEM). Then, all points that hit something other than the ground are skimmed off, leaving only the points that hit the ground surface. These are referred to as Bare Earth DEMs. A process is then used to connect points together with the same elevation, which creates the individual contour lines.

What is the format of the contours? 

Two formats are available: DXF and ArcView shapefile.

How is the order delivered? 

Your order will be copied to a CD or DVD, depending on the size of the order. A CD will be used if the total amount of the order is less than 800 MB. All orders exceeding 800 MB will be copied to DVD. Please note that you will not be receiving a paper map.

What is a .dxf file? 

DXF is a common file format that is used by AutoDESK's range of software. It can also be imported into other CAD packages such as Microstation.

What is a shapefile? 

Shapefile is an ESRI file format that can be used in most GIS software. It is actually is a set of three or more separate files (usually .shp, .shx, and .dbf) that work together.

How much data can be copied to a DVD? 

4.5 gigabytes

How much does each tile cost? 

2001 contours are $25 per tile and 2007 contours are $10 per tile.

What are the file sizes?

Unlike aerial photography, the file size of a contour tile varies depending on the format and the complexity of the terrain. The average file sizes are 8.6 MB for DXF and 1.9 MB for shapefiles. 

Where can I go to view or download samples? 

Samples of digital elevation contours are available in two file formats: Shapefile or DXF file.

What counties have contour files available? 

2007 contours are available for all 16 counties in the NCTCOG region.

2001 contours are available for Collin, Dallas, Denton, Rockwall, and Tarrant counties. There are also some in the northern part of both Ellis and Johnson counties, as well as some in the northwestern part of Kaufman County. Click here for a map of the coverage area.

How long will it take to process my order? 

Your order will be placed in the mail or be ready for pick-up 72 hours (3 days) from the time the order in placed.

Where can I view the specifications? 

Click here to view the specifications for 2007 digital elevation contours.

Click here to view the specifications for 2001 digital elevation contours.

What is the accuracy of the data? 

The elevation contours meet National Map Accuracy Standards for two-foot interval contours.

Are elevation labels (annotation) that appear on DFWmaps.com included with the contour files?

No. The DXF files have an annotation layer included. Shapefiles have an elevation field in the attribute tables which can be used to label the contours.  

Why are the contours on DFWmaps.com only shown at 10-foot intervals? 

Most likely the map is zoomed out too far. The 2' contours appear as you zoom in closer.