Evaluating Surveyor Needs vs. Wants
When discussing accuracy needs with surveyors, the magical +/- 1/10 foot requirement always pops into the discussion. This is a 1.2 inch (3.048 cm) so let’s just say 3 cm requirement. This is achievable with drone LIDAR/Photogrametry if ground control points are used and the data are debiased. However, do we really need this for common applications such as topographic mapping or do we just want it because surveyors always want the highest accuracy, irrespective of need?
Drone Based LIDAR/Photogrametry for Topographic Mapping
A surveying company was interested in using drone-based LIDAR for topographic mapping. A typical project could be a vegetation (e.g. tree) covered parcel being mapped for land development. The initial data model is used for overall site planning and initial clearing estimates.
They where asked how the work was performed without a drone. The answer is one or more of RTK, digital leveling or total station depending on what was appropriate for the particular situation. Well, we do not need to ask the accuracy of these devices – it is generally sub-centimeter relative to the local differential reference (e.g. base station) or monument. However, the big consideration is the nature of the terrain and the density of profiles or points to be collected. Drone LIDAR/Photogrametry is a very dense collection with an average nominal point spacing of 8 cm or even tighter. This means the terrain is saturatewith an average of about 150 points per square meter.
LIDAR Accuracy
Now compare this to the sampling that occurs in conventional surveying. If I could collect a point every 10 m(+/- 30ft), I would have an average sample density of 1 sample per 100 m2. Each of these samples would exhibit very high vertical accuracy (say 1 or 2 cm RMSE relative to a reference) but what happens between these samples? If the area is flat as a pancake, you are good to go. However, if you are working in rugged terrain, all bets are off. The terrain can vary dramatically between your samples, severely degrading overall accuracy. Ironically, the rougher the terrain, the denser you need your samples but the more difficult they are to obtain with conventional survey.
Conclusion
Many people get into arguments on how accurate something is without focusing on the end result. Using a half second total station to perform a topo survey does not ensure a highly accurate survery with high fidelity to the actual conditions. In other words by using LIDAR/PHOTOGRAMETRY you loose instrument "accuracy" but you gain orders of magnitude more final product accuracy and fidelity.
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