Nevada City Engineering

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Andrew R. Cassano, PLS
William D. Green, PE
John E. Baker

Daniel Hoagland

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Kenneth E. Baker Thomas L. Lott, PE


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About Aerial Topographic Surveys
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All About Aerial Topographic Surveys

Advantages:

Our firm can work in conjunction with an aerial surveying company to provide aerial topography of properties. Often this is the most economical way to obtain detailed topographic information over large acreages. Whenever topographic mapping is needed on parcels larger than about 5 acres, aerial topography starts to become more economical. On large acreages, aerial surveying can be a fraction of the cost of ground topographic surveying.

Typical Procedures:

We typically seek proposals from aerial surveying companies and select a company that we have worked with before that has a demonstrated track record for performance, price, and accuracy. The general procedure for aerial mapping is as follows:

  1. Our firm lays out targets or panels on the ground in locations specified by the aerial surveyor. These appear on the ground as a large white “X”s that are large enough to be visible in aerial photos.
  2. We survey the relative locations of these panels and provide their horizontal and vertical coordinates to the aerial surveying company. Our work is completed using conventional surveying equipment or with survey grade global positioning systems.
  3. The aerial surveyor actually flies over the property, taking a series of black and white photos with a large format aerial camera. Color photos can be obtained as an option, too. Using our panel information and the photos, the aerial photographer uses special photogrammetry equipment to compile topographic mapping showing contours, visible improvements, and foliage limits.
  4. The aerial surveyor typically provides their product in both hard copy and digital form. The digital format is compatible with standard AutoCAD drafting software. We usually work with this product to add additional information as necessary, like approximate or accurately determined property boundaries.

Aerial Survey Limitations:

Most topographic mapping that our firm provides is intended to meet National Mapping Standards. This means that at any given point on the map, the elevation shown or interpolated will be within ½ contour of being correct. So if the contour interval is 5 feet, then any spot elevation should be accurate to within 2.5 feet.

In heavily treed or brushy sites, this accuracy is not possible using aerial topography methods. Essentially, if the actual ground is not visible in the photographs, then the aerial surveyor cannot state with any certainty that the contouring is accurate. If buildings, roads, walls, and other improvements are not visible in the photograph, then the aerial surveyor cannot show them.

Often aerial topography of heavily wooded areas will include contours that are dashed to show that they are an estimate and that they do not meet the National Mapping Standards outlined above. Often buildings, roads, and other improvements are only partially shown and it is necessary to fill in this information using ground methods at additional expense.

If detailed information, such as individual tree locations are needed, this data must be gathered by conventional field surveys. Using the AutoCAD software, it can be superimposed on the aerial mapping. Generally, aerial mapping is often sufficient for broad brushed planning, but additional design surveys must be done later for the design of working grading and improvement plans.

We make every effort to disclose the limitations of aerial mapping before undertaking such a contract. We don’t like to see our clients disappointed when the level of detail that they were expecting just isn’t possible using this method.

Aerial Surveying Alternatives:

Often, we can work around the need for detailed aerial mapping for larger acreages, depending on the intended land use. One common way is by obtaining an uncontrolled (not to scale) color aerial photograph. We can add rough contours from the USGS maps for planning purposes. USGS topography may only have contour intervals of 20 or 40 feet. However, using this technique, we can usually get a feel for general slope, drainage locations, ridge tops, and so on. Buildings and roads have about the same visibility as they would in an aerial surveying product.

Once general planning is completed using the approximate information in conjunction with a field review and walking notes, we can fill in the detail in key areas, including major tree locations, marked underground or obscured features, and other information. The fill in work is used for detailed planning and working drawings, too.

Sample Mapping Recommendations:

We can help you evaluate the appropriate level of detail for planning and design work, based on your particular property and anticipated planning and design needs. The following table provides our general recommendations for typical projects.

Type of Project
Digital Aerial Photography and USGS Approximate Contours with Optional Ground Surveys as Fill-in
Ground Topographic Survey Locating All Major Trees and Ground Details
Aerial Topography with Optional Ground Surveys as Fill-in
Land divisions creating parcels of 1 acre and larger or smaller parcels on a site of 5 acres or less
XX
XX
XX
Land divisions creating parcels less than 1 acres in size on a site larger than 5 acres
XX
XX
XX
Master Plans or Specific Plans covering site larger than 5 acres and determining only general land use areas
XX
XX
XX
Commercial, Industrial, and Multi-family Site Plans
XX
XX
XX
Low to Medium Intensity Camps with Major Open Space Retention
XX
XX
XX
Master Drainage Plans, Preliminary Street or Utility Routing Studies
XX
XX
XX
Expansions of existing complexes, facilities, or utility modifications
XX
XX
XX

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