Quality Assurance – Field Checking Aerial Mapping

Accuracy standards create a way to define accuracy of a data set, but they do not define how to check and ensure that the data meet those standards, or that the proper level of detail is collected. We are constantly refining “how” to check aerial mapping to ensure stated standards while maintaining Cooper Aerial Surveys Co. level of quality and detail that has been previously expected.

For example, rough terrain will always be much more inaccurate than flat and “well defined objects” or spot elevations in flat areas that were held to twice the vertical accuracy in the past. Cooper Aerial Surveys Co. still holds this true.

The following are Cooper Aerial Surveys Co. aerial mapping checks:

Ghost Point Checks
Aerial triangulation will be checked by establishing ghost panels independent of ground control. These points will act as unknown control and will be read into the analytical solution. Once calculated, these points will be provided to the project manager and checked against the field results. This accuracy verification will determine readability (RMSE) of the flight and act as a preliminary check of the photogrammetry before photogrammetric data compilation. Upon approval each point will be incorporated into the analytical solution for a stronger solution.

Cross Section Checks
Accuracy will be checked by collecting cross sections in the field which will be compared against the DTM produced by the aerial mapping photography. Cross section checks will be conducted according to the current FEMA Guidelines A.7.5.4. In addition, each cross section will be collected perpendicular across all major drainage features. Cross section checks should be an accurate representation of the ground detail and test for the proper amount of breaklines used by the aerial mapping software for accurate modeling. Ground surveyed cross sections must be at least 6” long at final aerial mapping scale. Surveyed points point should reflect each break in the terrain greater than ¼ contour, at an intervals no farther than ½”, and no closer than 1/20” aerial mapping scale.

Random Spot Check
Accuracy will be checked by collecting random point clusters of each ground cover category which will be compared against the DTM produced by photogrammetry. Well defined objects will be used when possible. All points checked will be used to meet the RMSE requirement established by NSSDA and FEMA guidelines A.4.3 and A.8.6.2. Like ghost points, all check points collected in flat areas, principle road intersections, railroad intersections, well defined object, and spot elevations will be held to twice the accuracy of the DTM checks.

Depending on the project we typically use: one ghost point, AND one cluster of random spot checks or one cross section, per aerial satellite mapping sheet, or square mile, or linear mile. We either let the surveyor or QC manager ensure that the data collected in the field reflects the data that should be collected in the aerial mapping.