Aerial Mapping and Survey Accuracy
Drone aerial data can be an incredibly powerful tool, but as with other technologies it is important to use it in the right way and with some knowledge about its strengths and limitations.
Falcon has surveyors on staff and we understand the challenges and limitations of aerial data. If you have any further questions on this topic, please feel free to get in touch and we’ll get you an answer and also update the information here to cover it.
Mapping-Grade vs Survey-Grade Accuracy
To start with it helps to consider two broad categories of aerial data accuracy:
Aerial Mapping produces results which are visually accurate.
Aerial Survey produces results which are measurably accurate.
Mapping-grade accuracy uses only the drone’s on-board GPS for positioning information, and so tends to be accurate to a few metres horizontally but not in terms of elevation. Mapping-grade data is suitable for things like vegetation and environmental assessment, a visual site plan, or any other purpose where you only need to ‘look and point’ at the map, count things or assess relative sizes rather than taking precise measurements or coordinates. These outputs look correct to your eye, but if you were to take measurements from the map and correlate them with measurements on the ground you would find there can be some quite large inaccuracies (typically several metres horizontally, and even as much as tens of metres vertically).
Aerial Mapping example: an overhead ortho-mosaic map, which in many cases does not require survey accuracy
Survey-grade data is required in cases where accurate distance, area, elevation and volume calculations are required, such as Digital Elevation Models (DEM) or point clouds for stockpile volumes, drainage design, site planning and engineering projects. For typical survey projects the accuracy of aerial drone data is +/- 50mm lateral and 50-80mm vertical (though any range of accuracy is possible, see ‘Variable Accuracy’ below).
Aerial Survey example: by using GCP & careful data processing an accurate Digital Elevation Model can be produced
To obtain survey-grade data it is necessary to first place Ground Control Points (GCPs) in a grid around the site and to measure these positions to <2cm accuracy (which requires professional survey equipment, not a normal GPS or your iPhone!). These visual targets are then captured and tagged in the aerial imagery during data processing so that the resulting DEM is tightly fitted to these positions, ensuring accuracy throughout the survey area.
Placing and surveying the GCPs can often take us more time on site than is required to fly the aerial data capture, but is a necessary step to both ensure and validate the resulting data accuracy. Over the past 5 years AUAV has built up a good working knowledge of how much ground control is required and where best to place them in order to guarantee the accuracy of our results.
Variable Accuracy (i.e. anything in between)
As well as our standard 50mm survey accuracy, it is possible to push this to greater precision in some cases. Accuracy of 20mm or better is possible, though it tends to only be feasible for small areas (e.g. engineering projects such as a bridge or tower, i.e. up to a few hundred meters in scale).
Likewise, if you require less precision but still need some degree of known and verifiable accuracy then it is possible to use sparser ground control, for example to achieve a 25cm accuracy which is suitable for the 0.5m contours typically used in flood modelling and similar purposes.
What tends to work best is to discuss your accuracy requirements with us (or any suitably qualified aerial survey company) so that we can best fit the precision to your needs. There is a direct trade-off between accuracy and cost, so it literally pays to not over-specify accuracy requirements.
Some applications of aerial photogrammetry include:
- Topographic mapping
- Precise mapping of building outlines and roof structures to create 3D models
- Volumetric determination of stockpiles and mine voids
- Preparation of Land Use maps