Digital Terrain and Elevation Modeling (DTM and DEM)
Data sets that represent elevations on the earth’s surface, DTMs and DEMs are tools instrumental to the creation of reliable geospatial deliverables showing an accurate representation of surface features.

Digital terrain models generally define surface features in an irregular spacing pattern, using both breaklines and data points to show abrupt elevation changes in detail. The result is a more accurate depiction of the terrain, which is useful for TIN calculations, generating contours at small intervals, making volumetric calculations, and ortho-rectifying aerial images. DEMs, though similar to DTMs, define terrain features in a regular, or grid pattern, so that changes in elevation are smooth, gradual and less detailed.

DEMs are primarily used in the ortho-rectification process, GIS applications and the generation of contours at large intervals. Based upon the requirements of the client, DTM and DEM data sets can be collected as part of a larger photogrammetric service or delivered unto themselves as engineering design tools.  

Sample Project:
Project 03043 I-95

Major Maryland DOT project to widen highways I-95 and I-695 North of Baltimore from 4 lanes to 6 each bound using multiple engineering and construction firms. Maryland State Highway Administration is supporting technology improvements in mapping, drafting and project management.

Mapping requirements for a 1200' corridor design along highway I-95 involved aerial color photography, using a flight height of 1200' above ground level, to produce a map scale of 1” = 40' and developing a 20' DTM grid to generate 1’ contour interval. In addition, the aerial photography was used to produce digital color orthophotography and seamless orthomosaics. Final products included MicroStation V-7 and V-8 digital drawing files, georeferenced orthoimages and various supporting data.

The approximately nineteen mile project required 220 color images within 10 flight lines and used 132 ground control panel points. In addition to normal requirements, the planimetric drawings included all above-ground utility data, traffic flow lanes, and supplementary bridge deck details.

Information collected and provided by TAS was also used to perform noise studies, drainage design, volumetric analysis, earth movement computation and support Geographic Information Systems analysis.

Sample Project:
Project 05001 US 17

The project is located along the east coast of South Carolina between Charleston, SC and Savannah, GA. Ongoing and intensifying safety concerns have initiated the project which involves improvements to approximately 22 miles of US 17 from Gardens Corner, SC to Jacksonboro, SC through the ACE Basin. The project involved 500’ corridor mapping along US 17.  Digital Terrain Models were created at a 25’ grid along the project area and supported both contour generation and the rectification of orthophotography.

Duties include:

• • NGS or SCGS control points used to set control pairs throughout the site using GPS static surveys
• • Conventional ground survey methods used to locate edge of pavement, centerline, and ditches
• • Established horizontal and vertical control
• • Use topographical information to develop cross sections at 50-foot intervals on curves and 100-foot intervals on tangents
• • Public records research of all properties that might be impacted by the construction of proposed project
• • Produced a base map in Microstation v8 format
• • Acquired aerial photography of the mapping area.
• • Produced photogrammetric design mapping in MicroStation v8 format.
• • Produced a Digital Terrain Model using photogrammetric techniques.
• • Produced orthophotography of the project area.

Sample Project:
Project 07050 GADOT

This service was provided to the Georgia Department of Transportation.  A corridor of US 27/SR 1 in Rome, Georgia, was to be collected stereoscopically at a map scale of 1”=50’.   Control points were placed and collected; these points were then measured along with pass-points to run the aerotriangulation software and obtain an accurate solution.  Topographic data for the job was collected 800’ from the centerline each way along the north portion of the project area heading into town.  The south side of the project area was collected with an additional 300’ on each side of the road to ensure sufficient detail.

Once fully collected the compilation was used to create a DTM for the purpose of ortho rectification.  Black and white photography was rectified using the DTM with a ground sample distance of ¼ of a foot.  The imagery was formed into a mosaic and used as an overlay for the compilation as a method of quality assurance.

Sample Project:
Project 08012 PTAA

This is a project of the Piedmont Triad Airport Authority to support its long-term planning and development activities. A plotted image showing the immediate vicinity of the airport is used to record on-going roadway, parking and runway developments, while another image, 8.5 feet by 15.5 feet is used to investigate and demonstrate flight paths and noise issues.

This 64,000 foot wide by 100,000 foot long orthophotography project surrounds the Piedmont Triad Airport in Greensboro with the long axis parallel with the primary runway. Bare earth LIDAR data grided to 50 ft was obtained from the North Carolina Flood Plain Mapping Program. LIDAR data was used in conjunction with photogrammetrically collected DTM and breaklines to rectify photography at 2 foot pixel resolution. Two deliverables were required, the first being a 20,000 foot by 20,000 foot photo image of the area surrounding the airport proper, the second deliverable being, a photo mosaic of the entire area, printed on photo paper and laminated to preserve the paper and ensure the accuracy of the print. Fifty six digital color orthophoto tiles, each 10,000 foot square, were also produced and delivered on DVD media.

The client required that TAS provide color aerial photography using high-resolution, distortion-free, six-inch focal length camera with forward motion compensation. The photography was flown at approximately 15,000' above mean terrain to achieve an approximate photo scale of 1”=2,500' with 60% forward overlap and 30% side lap. Four parallel flight lines of 12 exposures each were acquired, covering a rectangular area of approximately 12 miles by 19 miles.