When you produce a 3D model using photogrammetry, you have the option to select what coordinate reference system (CRS) it is generated in. 

A CRS defines how your model is related to real places on the surface of the earth. As the world is not a uniform sphere and is covered with valleys and mountains, each region will have its own CRS, which provides the most accurate representation of that patch of earth. 

How can my coordinate system be wrong?

No coordinate system is technically wrong, it's just not correct for the purpose of what you are trying to measure. A coordinate system can be wrong by being different to external data you want to measure to, i.e. design files provided by an engineer or survey benchmarks provided by the local government. 

Alternatively, it might be wrong compared to the last dataset was produced for that location. The type of CRS mismatch will have different impacts on how you view and measure with your data.

Types of mismatches

Differing vertical systems

If you have different vertical systems it will cause a vertical shift of your model compared to a previous dataset or to external GIS data. Vertical mismatches are often due to confusion over similarly named geoids. 

Differing horizontal systems

If you choose the wrong horizontal system it may be hard to tell that anything is wrong. If you make measurements of stockpiles or calculate the slope of a road grade, everything will appear normal. However, if you start comparing it to an external design file or a previous model you will notice a horizontal shift between common features. 

Differing horizontal and vertical systems

If both the vertical and horizontal systems don't match, it's very likely there will be significant shifts between your surveys. Luckily, depending on how different the systems are, it is likely that this would be detected by our quality assurance checks. 

I still don't understand!

We wrote these articles to arm you with everything you need to get the job done on your own, but we understand that sometimes this isn't sufficient.

If you're stuck, the Propeller data success team may be able to help. You can contact our support team by clicking the support tab on the top right pane of your user portal.

 

Did this answer your question?