Digital Elevation Models – Data Meets Art
Digital Elevation Models (DEM) are a very effective way of visually communicating the height of an object. As the colours shift from red (shallow) to blue (deep) the elevation or depth of an object like a wreck on the seabed is quickly understood. With the photogrammetry survey of the SS Thistlegorm extending over 5 acres we can see exactly how the depth water over the ship changes:
DEM of the SS Thistlegorm. The 5 acre survey area clearly shows the port and starboard trains.
As clearly shown by the shifting colours, the wreck stands out like a sore thumb on a very gently sloping seabed.
To generate a DEM we need the following:
- A 3D object such as a sparse cloud, a dense cloud or the mesh surface itself.
- The 3D object needs to be referenced.
For the SS Thistlegorm the 3D mesh – the surface itself – has been used to generate the DEM. GPS points were gathered by the team and used during the build of the model, giving us a GPS (WGS 84 to be precise) reference to work from.
There are two ways of representing the DEM and personal choice is always to use something called hillshading mode. If you would like to know why, please check this link where all is explained.
Visually, DEMs are very alluring. They really do combine data and art in a single view. For me, DEMs represent the ideal expression: inherent visual expression the eye can consume, interpret and understand and appreciate in a single pass.
The Rope Room – the enclosed area under the forecastle – is a DEM that I’m drawn to again and again. The coils of wire hawser stand off the deck very clearly.
The rope room under the forecastle deck on the SS Thistlegorm. Research has now revealed the purpose and use of the three smaller rooms.
Another favourite is deck two:
Deck 2 of the Thistlegorm’s forward holds.
The Published DEM
The DEMs will feature in the forthcoming paper that will be published in due course, documenting the survey. Such is the accuracy of the DEM, it is possible to see the damage the wreck is sustaining by trapped exhaled breathing gas. Using DEMS in this way should help others -non divers – understand the risks the wreck faces today.
Such is the beauty of DEMs one features on a 1000 piece jigsaw of the Thistlegorm.