OptiSurface Designer can import topography data from aerial and drone surveys that use LIDAR or photogrammetry. However, we are cautious about using this for landform designs because the vertical accuracy of this data is typically +/-50mm or -+2 inches.
For ditching, surface drains or tile/pipeline applications +/-50mm is probably fine. However, OptiSurface landform designs typically move less than 50mm or 2inches on average over a field. Therefore, if the survey data has this level of inaccuracy to start with you could end up moving 200% more soil than required.
There can also be coordinate system differences between the topographic data and the machine control system which can introduce horizontal and vertical biases or offsets between the survey and machine control system (see Coordinate System Differences with Different GPS Systems).
Therefore, currently we recommend topography data from aerial and drone surveys can be used to do preliminary landform designs, drainage analysis, runoff analysis but not for final designs.
If you can get the data into a text file with the format of x, y, z then OptiSurface will import it. However, LIDAR and photogrammetry data is often very dense and OptiSurface Designer can filter down the point number to 30,000 per field to ensure faster operations/calculations.
Aerial Survey Accuracy
This is copied from the Sensefly (leading drone survey company) FAQ
Q: What kind of positional accuracy can I expect?
A: The accuracy of the orthomosaics and the digital surface models (DSMs) produced by professional photogrammetry software such as Pix4Dmapper (optional with every senseFly drone) depends on various factors, including: flight height, light conditions, availability of textures, image quality, overlap, and the type of terrain being mapped.
In standard conditions, flying an eBee or eBee Ag at an altitude of 100-150 metres (328 - 492 ft) above natural terrain, with an image overlap of between 50% and 70%, and not using any ground control points (GCPs), a relative accuracy of 10 cm (3.9 in) and an absolute accuracy of 1 - 5 m (3.3 - 16.4 ft) can be expected at the location of the found matching points, when using Pix4Dmapper. Between these matching points the accuracy may vary.
These figures are vastly improved by using GCPs, with absolute accuracy then possible of down to 3 cm (1.2 in) / 5 cm (2 in).
As for the eBee RTK, this survey-grade mapping drone is capable of achieving the same absolute accuracy - of down to 3 cm (1.2 in) / 5 cm (2 in) - without the need for GCPs.
Read the eBee RTK Accuracy Assessment whitepaper.
(Please note that senseFly cannot guarantee in any way the quality and accuracy of your output files.)
This video shows how to use Ground Control Points. It would be also useful to use points collected as you drive between the GCPs as independent checkpoints as explained in the video below:
Aerial Survey Accuracy Test
If you wish to test the accuracy of your aerial survey with ground based survey we recommend a procedure like this:
- We recommend covering an area at least 1km x 1km.
at least 10 control points over the area. One at each corner, one a the
center and then the five more spread over the field. There should
probably be 9 (3 x 3), 16 (4 x 4) or 25 (5x5) ground control points
depending on field size.
- Surveyed these control points as
benchmarks on the Trimble Field Level 2 system or the system you are
planning on conducting the earthworks with because they can use
different horizontal and vertical coordinate systems.
the Sensefly post processing, you would need to get the x, y, z of
these same points. Then in a spreadsheet we could compare the x, y, z.
- Do not carry out any correction/adjustments to the Ground Control Points but check the difference in x, y, z at each point.
- Then carry out a survey adjustment on the aerial survey with half the Ground Control Points, then the difference in x, y, z at the remaining check points.
Note that the Trimble Field Level 2 System uses a slightly different coordinate system and transformation from another coordinate system could produce some errors. See this article for more info: Coordinate System Differences with Different GPS Systems