Accurate Capture

Creating a 3D model using photogrammetry can be pretty straightforward. Avoid the usual pitfalls of image quality and overlap and as if by magic a 3D rendered shape appears.

But what if you need accuracy? Or the client demands it? In our view, two sorts of accuracy exist:

  • Relative accuracy – how close to reality are measurements taken in the scene?
  • Absolute accuracy – whereabouts on the planet is the scene?

Without scale the model is a pretty blob and has limited use. There’s not much to be done apart from look at the model, which for some is enough.

To add value to the 3D model we need to force accuracy. So scale bars and real world dimensions are added and correctly applied the model starts to have relative accuracy, and measurements taken within the scene can be very reliable.

Bunkers, known a pillboxes make great test shoot models. This example is built into a Bronze Age burial mound dating from 4000~700 BC.

Add in some known GPS points and the context of the rest of the planet is added. Absolute accuracy depends on the quality of the GPS points; an iPhone might be ±5m and a survey station ±1mm. Both come with their own price tag.

But what if you could shoot, capture GPS and get accuracy that meant your scale bars become check bars?

Accurate GPS

Check bars are used to verify the model, not force scale, and they serve to give confidence in the end result. Check bars are needed and in this example they were added early in the model, just after image alignment. In this instance GPS reference data was added before alignment and used in the process. When the check bars were added the accuracy was – to put it bluntly – stunning. This model has not only relative but absolute accuracy all in a single model and at first alignment.

A closer view of the Reference tab in Metashape the top red circle is the Estimated Values button and the lower red circle shows the estimated error of the check bars.

No refinement or updates were applied to the image alignment, they are as-is from alignment alone.

From initial accuracy like this things only get better. Refining alignment dropped these values a little but the actual differences were measured in single digit mm. With accurate data from day one everything flows out of the batch process with minimal effort.

Orthophoto and DEM

After the initial model was complete the now-obligatory orthophoto and DEM were produced. The orthophoto is hosted by those nice folks at DroneLab:

The DEM clearly shows where the ancient barrow was dug into to site the pillbox. Since it was constructed in 1940 the pillbox has been repurposed and now acts as a base for a mobile phone mast.

The DEM clearly shows the area dug to sink the pillbox into the barrow and the four higher areas are the base of a mobile phone mast.

Summary

So how did we get such accurate GPS? That requirement has been driven solely by working on an ongoing project with law enforcement agencies. For now how this was achieved will not be revealed, but once refined and ready all will be revealed.