Photogrammetry Measurement and Scale

A Pretty Blob

Photogrammetry is an excellent way to capture the world around us. Anyone can get involved and explore a subject that might otherwise be out of reach. The underwater world is such a place and with just 0.5% of the population able to dive bringing a shipwreck to the surface in digital 3D is a great way to show the world what divers see.

But when you dig into the detail 3D models reveal their limits. This is brought into sharp focus when the field work ends and the research begins. Measurements are needed to verify and test theories or to support claims. If you have an unscaled model then you have a 3D representation but its usefulness is limited. Its just a pretty blob.

Adding the Value

As soon as you start to ask ‘How big/long/deep’ a scaled model is needed. As the research into the SS Thistlegorm progresses the value of measurements and GPS marks recorded by the team reveal their value.

One artefact that has been located are a copy of the builders plans. With the actual ‘as intended by the builders’ measurements to hand we can verify the model. For example, the builders plans state Hold 1 is 20′-0″ x 29′-3″ (6.096m x 6.1722m). With the model loaded in Photoscan Pro we can quickly measure the same distance:

Screenshot showing the measure tool in Photoscan Pro recording the distance across Hold 1

Without labouring the point, when checked against the builders plans the measurements are consistently confirming the model scale.

Anywhere & Anytime

Photoscan Pro generates some wonderfully detailed ortho photos and in the case of the SS Thistlegorm the cargo holds have been rendered at a scale of 1mm = 1 pixel.

The good news is Photoscan Pro allows us to measure directly from the ortho photo and between any points as we desire. Here’s an example of a measurement of one of the crates of Lee Enfield rifles on the lower deck:

Screenshot showing the length of one of the crates of Lee Enfield rifles. The overall length of a SMLE Mk III is 1.130m

This is the value of scaling. Measurements can be taken from the model, the ortho photo or the DEM.

3D blobs can be interrogated long after the diving has ceased. Scaling and GPS adds value and when sat in front of a computer the ability to interrogate the model and ortho photo in this way is a huge advantage. The ortho photo can include things that are normally lost due to movement so why not measure the length of a fish? Lower deck lorry width is probably more useful:

Width of the load bed of a light truck complete with Norton motorcycles. The fish add an arty touch.

So What?

As much as we would all love to dive the SS Thistlegorm again its simply not cost effective. Some measurements – such as the overall length – would be very difficult to actually measure in the water. Any arbitrary measurement no matter how daft can be checked:

The distance from the centre of the funnel to the centre of the locomotive chimney is rather abstract but we now know its 29.8m between those two points.

Being able to measure in the comfort of the computer is extending what we know about the wreck – watch this space for more news.