DTM   As defined by this project, a DTM is similar to a DEM in that it contains points which are represented as x,y,z coordinates. In the DTMs however, these points have an irregular distribution, are fewer in number, and include only significant break lines due to their use solely for imagery rectification.

To correct for distortion in the aerial photography, DEMs/DTMs were used as control points for the CAPCO region. These were compiled through a combination of airborne and ground, control point generation. The DEMs contain break lines for such features as streams and ridge lines and are identified by the smaller grid cells containing a seven digit number followed by a letter. The DTMs have fewer mass points and break lines and while they have good relative accuracy their absolute accuracy is unknown! They are noted in the provided locator grid as seven digit numbers within the larger grid cells. To find the DEM/DTM you want, you will need to examine the zipped grid, provided in .dgn, .dwg, and .dxf formats, at the link below. All data was generated in NAD 83, State Plane, Texas Central projection, Feet.

These DEMs/DTMs are a byproduct of the imagery development process and as such were not used by the GIS section at CAPCO. Use them at your own risk as they have not been verified for accuracy or completeness. The data here is provided in a sequential format of around 100 models per zip file. To look at them you will need terrain modeling software which utilizes ASCII format files. Because of the variances in the data representation, it is strongly suggested that these files be USED FOR GENERAL PURPOSES ONLY AND NOT FOR CONTOUR GENERATION. They are well suited for creating a generalized 3D model of the CAPCO region as individual tiles or in mass.

Feature codes:

25    "paved road centerline"   28    "unpaved road centerline" 321    "extended job boundary" 322    "low flown map boundary" 325    "photo job boundary" 339    "contour breakline"

345      "mass point" 346      "breakline sharp" 347      "breakline round" 348      "ridgeline" 349      "extrema"

Format key for *.xyz ASCII files:

• Feature code/element types ending in "1" are LINE types.
• Feature code/element types ending in "2" are ARC types.
• Feature code/element types ending in "5" are POINT types.
• The number preceding "1" "2" or "5" is a feature code. (see below)
• The point # refers to the point, or record number in the line string or point.
• Road Centerlines should be treated as extrema.

Example Data:

Feature Code/Element Type	Point	X	Y	Z
281	1	3274187.83	9886560.77	346.22
281	2	3274206.86	9886489.35	345.18
3455	1	3274225.83	9886422.68	344.06
3461	1	3270495.50	9888483.44	407.90
3461	2	3270333.45	9888308.90	416.30

The image at right shows the locator grid and the two areas of coverage for 1997 DTMs (purple) and DEMs (black). Click on the image at right to get a better graphic of the 1997 DTM(red)/DEM(green) coverage area. For more information contact the GIS Department by clicking here

 

CAPCO is providing this data free of charge as a service to the community. CAPCO obtained the data from sources believed to be reliable, and it has made reasonable efforts to provide accurate and complete data. The user acknowledges, however, that the data may not meet the user's needs, may be incomplete, may contain errors or omissions, and may not be up to date.

CAPCO DOES NOT WARRANT THE ACCURACY, TIMELINESS, OR COMPLETENESS OF THE DATE, THE MERCHANTABILITY OF THE DATA, OR THE SUITABILITY OF THE DATA FOR A PARTICULAR PURPOSE. CAPCO IS NOT RESPONSIBLE FOR MISUSE OF THE DATA.

 

TOP OF THE PAGE