Topographic and Planimetric Mapping

LiDAR with contour linesDigital mapping systems contain embedded intelligent data that can be visually interpreted by the user. This data, as part of a Geographic Information System (GIS), can serve many applications. Utility companies can use it to catalog their infrastructures, assessors can use it for property assessment and taxing purposes and engineers can use it to aid in planning upcoming projects. Typical applications include:

  • Engineering
  • Energy exploration
  • Natural resource conservation
  • Environmental management
  • Public works design
  • Commercial and residential planning

Digital orthophotography, a seamless, highly-accurate image of the Earth’s surface, forms the base of a map, and linear data (vector) – topographic contour lines and planimetric lines – can be overlaid to add useful information. With an interactive GIS map, Users can zoom in and out of an image, pan in all directions, and see/hide individual “layers” of mapping data with a click.

See an example of an interactive map.

Topographic Mapping

Topographic contour lines without digital orthoimagesTopographic maps depict the three-dimensional shape of the earth’s surface with contour lines at specific intervals to depict the shape and elevation (or relief) of terrain. Topographic maps often portray both natural and man-made features.

Contour intervals are determined based on a client’s specific needs. The smaller the contour interval, the more accurately the map reflects the terrain detail. The purpose of the contour will determine the required contour interval:

 

 

Contour Interval General Purpose
1-foot Final design, excavation and grading plans, earthwork computations for bid estimates, and contract measurement and payment
2-foot Route location, preliminary alignment and design
4-5-foot Preliminary project planning, hydraulic sections, rough earthwork estimates

USACE Engineering and Design Manual for Photogrammetric Production

To ensure accuracy of the contours, the data must be collected at specific photo scales (minimum negative scale). The following table shows the Contour Interval in feet and the values that would need to be used to achieve a product accuracy of ASPRS Class I and II as well as NMAS.

Contour Interval ASPRS Class I NMAS ASPRS Class II
0.5-foot 1″=167′ 1″=175′ 1″=183′
1-foot 1″=333′ 1″=350′ 1″=367′
2-foot 1″=667′ 1″=700′ 1″=733′
4-foot 1″=1,333′ 1″=1,400′ 1″=1,467′

USACE Engineering and Design Manual for Photogrammetric Production

In brief, the smaller the contour interval, the lower the photo scale; this results in an increase in the project schedule and cost.

Planimetric Mapping

Planimetric lines without a digital orthophotoA planimetric map consists of both man-made (cultural) and natural features. These features are represented three-dimensionally as lines (vectors) and points. These features are extracted from aerial photography using modern technology; anything visible in the imagery can be digitized. Common features include:

  • Street and water centerlines
  • Sidewalks
  • Culverts
  • Utility lines
  • Building footprints
  • Vegetation

Planimetric lines overlaying a digital orthophotoUseful data can be attached to specific geographic features as part of this intelligent map; for example, in county assessors can embed information about a property’s title, owner, assessed value, and so on. As such, the map / Geographic Information System becomes a storehouse of valuable data that can be quickly and easily accessed.

When constrained by budget, many clients decide to acquire digital orthophotography and/or planimetric features only. The following table illustrates the minimum negative scales (photo scale) needed for Digital Elevation Model Extraction and planimetric mapping.

As you can see, the overall project cost would be significantly reduced by eliminating the contour component from the mapping. The most important item when selecting mapping features to be produced for a project is to take careful examination of usefulness and necessity of certain types of data, such as 1-foot contours.

Target Scale 1″ = x ft. ASPRS Class I NMAS ASPRS Class II
50 1″=350′ 1″=375′ 1″=400′
100 1″=700′ 1″=750′ 1″=800′
200 1″=1,400′ 1″=1,500′ 1″=1,600′
400 1″=2,800′ 1″=3,000′ 1″=3,200′