CAPCOG GIS 


      
       DEMs/DTMs 
       
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      DEM   As defined by this project, a DEM is a grid of equally spaced points 
      which are represented as x, y, z coordinates. They include break lines and 
      can be used to generate 5' contours. 

      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 YZ
            28113274187.839886560.77346.22
            28123274206.869886489.35345.18
            345513274225.839886422.68344.06
            346113270495.509888483.44407.90
            346123270333.459888308.90416.30


       
      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.
       
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