Mapping with the Mavic

Aerial mapping with the Mavic 2 Pro for 1:200 sale topographic survey is possible and the theoretical GSD is adequate. As the Mavic has no DGPS capacity GCPs are essential. Issues of image resolution, GPS/barometric heighting* and autonomous mission control have all been discussed elsewhere on the web. Given the comparative low cost of the Mavic compared to platforms with far better image reso potential (Inspire, Matrice etc.) I post this example to show what can be done with the ‘pro-sumer’ unit as a topo survey tool.

To attempt optimal cover the site was flown in 2 campaigns comprising multiple swaths including some low level manoeuvrers under the tree canopy in August and a single swath flown in December. Photography was taken with the drone static before exposure due to the known risk of electronic shutter blur on images captured with the drone in motion. The KAP trick of pushing the shutter speed to kill blur doesn’t work so well with the Haselblad L10-20C. The operation was conducted in accordance with the ‘standard’ permissions granted under PfCO.

Ground control was a simple base line between two natural points measured by TST.

The path of the 50m AGL swath.
Flight duration for this path was 11m53s with a max speed of 16mph with phtography taken at 3mph in 5 frame burst mode.
617 frames captured by the Mavic 2 Pro under PfCO standard permissions, processed to generate 443,645 points. The camera alignments show the capture strategy: a block at high level (50m AGL) and a number of obliques in an attepmt to infil the ground cover obscured by the tree canopy.
The dense point cloud model. The surface is compromised by the tree cover.
Orthophoto as projected to a ‘height field’ 2.5D surface.
Aproximate GSD for the high level block is 1.18cm/px.
The orthoimage is scaled to a TST measured GCP baseline.
A repeat flight was flown in December to achieve infil with less leaf cover.
Ortho-image from 2nd flight, low contrast and reduced vegetation in December.
Digitising in 2d from the orthoimage in CAD. There are gaps!
Oblique imagery processed in patches for infil under the trees. Digitising from screen shots of the dense point cloud avoided the processing time for surface generation.
Infil shown in blue.
Model reprocessed with trees stripped from the point cloud.

The sparse point cloud was extracted to local co-ordinates (if full geo-reference co-ords are used to extract the points they conflict with CAD limits).
The datum was from flight data logged at www.airdata .com which recorded a height derived from 15 satellites for the TOLP. Contour generation is by TheoContour with manually splined polylines.

Having digitised as much detail as possible from the ortho-image a few more lines could be gleaned by returning to the the raw photography, the oblique views under the tree cover recovers one or two hidden lines.

Left: ortho image. Right oblique view showing under tree detail.

The plotting tolerance for for 1:200 scale is roughly 100mm on the ground. A GSD of 10.18mm represents 0.05mm at scale. The GSD of 1.18cm/px is derived from:

  • Sensor size: 13.2mm x 8.8mm
  • Image size: 5464px x 3640px (20mp)
  • focal length: 10.26mm
  • Flying height: 50m

Provided GCP are used and flying height is carefully limited the Mavic 2 Pro delivers useful cover for 1:200 mapping.

GCP vs Mavic GPS derived position. From photogrammetry the ground co-ordinates, based entirely on the Mavic Exif values, were seriously adrift from the measured GCP value:

Without a differential method the Mavic GPS cannot be relied on for mapping without GCPs. Operators have reported anything inside 10m can be considerd ‘good’ for the Mavic.

*The heighting issue is resolved with the firmware update to 01.05.10 on 16.09.2019.

Metashape https://www.agisoft.com/

Airdata https://airdata.com/

TheoContour http://www.theolt.com/web/theo-contour/

Happy surveys,

B

About billboyheritagesurvey

Heritage worker
Gallery | This entry was posted in SUA photography, Survey Practice. Bookmark the permalink.

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