Reporting Horizontal Errors in SfM Mapping

Fully appreciating the powerful role played by the Structure from Motion (SfM) technique in the mapping revolution, we understand that embedded in the fiber of an SfM produced map are some very important quality characteristics which, when not specifically disclosed and effectively displayed, will remain hidden to the user of the map. These characteristics can be understood as the quality DNA of the individual map. A responsible map maker should thus proactively determine and display this DNA for each of the maps he or she makes, thereby providing the individual map with an authentic certification that is much more relevant and meaningful than the credentials of the map maker himself.

One way to display horizontal errors is to plot them at a much larger scale than the map itself, as shown in the figure below.

Horizontal Error Visualization

The advantage of this type of error visualization is that it would very efficiently expose bias in position or rotation across the mapping domain. The disadvantage is that it is not all that easy to visualize the magnitudes of the errors.

So we are looking for effective ways to attach a clear picture of the true DNA to each of the maps we make. In the figure below there is another example of error reporting that may communicate better than numerical tables of errors or statements regarding the “class of accuracy” as defined by some or other organization. Here we show the “true” location of check points – determined to 10mm accuracy by total station – superimposed as yellow dots over the corresponding targets in the Ortho Photo. The distance between dot and target center represents the local horizontal mapping error.

Computed Errors

For a realistic understanding of the significance of these errors all the reader has to know is that the square targets have dimensions 20cm x 20cm and that the round white targets have a radius of 5cm.

Aren’t these “error-pictures” much more realistic and relevant than all the other ancillary, albeit interesting information such as the flying height of 50m, the wind speed of 5m/s, the GSD of 13mm, the MAP-M4 multirotor platform, the airframe mounted Sony a6000 camera with 14mm lens that was connected to a dual frequency on board V-Map GNSS receiver weighing only 130g (, the average estimated 3D camera position accuracy of 3cm and the combined number of more than 50 man-years of experience in mapping under the belt of the map makers?

Oh, and never mind the fact that no Ground Control Points were used to make this map!

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