How PIX4Dmatic automatically corrects measurement distortion
While most GIS and photogrammetry software requires a user to manually manage distortion by choosing appropriate projections or running specific tools, PIX4Dmatic has a built-in, automatic feature to handle this. The Distortion-Aware Measurements feature adjusts for the differences caused by projection distortion, ensuring your measurements are as accurate as possible.
The information in this article was provided by Lukas Juschka
Why are my measurements distorted?
Projection distortion
All map projections, by necessity, distort the world. Think of trying to flatten a spherical orange peel—you can't do it without stretching or tearing it. Similarly, to represent the curved Earth on a flat map, we must make a trade-off.
Some projections preserve the true area of landmasses (like in the Wagner IV projection, where all circles have the same size), but they stretch and distort the shapes.
Other projections, known as "conformal" projections (like Mercator), preserve the correct local shapes and angles, but they dramatically distort areas and distances, especially the further you get from the equator.
This is why accurately geolocated points that are, for instance, one meter apart in the real world will not necessarily be one meter apart on a projected map. The choice of projection determines which type of distortion you get. For technical work like surveying, it's crucial to understand these distortions because they directly impact the accuracy of your measurements in the 2D plane.
PIX4Dmatic accounts for this by using a specific projection (that is, a coordinate reference system or CRS), but the fundamental distortion remains.
How large is the effect?
In a good coordinate reference system for surveying, the effect is small, but it is still noticeable. For example:
However, if the CRS is not good, the difference becomes more drastic:
What makes a CRS "good" or "bad"?
A combination of different effects makes a CRS "good" or "bad".
- Each coordinate system has a different "area of use", which restricts the area surveyors are supposed to use this CRS in. Some coordinate systems have very large areas of use, while others are extremely specialized.
If the area of use is smaller, the parameters of the coordinate system can be more fine-tuned to work well for this location. For that reason, the US is introducing lots of small coordinate systems all over the country next year.
The area of use of EPSG:3034, meanwhile, is enormous, which is (at least in part) why the distortion factors for it are bad.
Different projections have different other properties, which serve different needs and uses or industries, for example, navigation (naval or aviation), and some are better for surveyors. As a surveyor, ideally, you want to use a projection that has low distortion factors in your area. In geospatial software, it’s a common practice to assume that a meter in a projected coordinate system (CRS) is equivalent to a meter in the real world.
Distortion-aware measurements in PIX4Dmatic
With PIX4Dmatic’s distortion-aware measurements, you have real-time corrections built into the software! If the setting is on, every measurement of a distance/area/volume you see in the application is adjusted. The feature accounts for these distortions to ensure measurements match the real world as closely as possible.
Here's how it works:
- For outputs: When enabled, every measurement of a distance, area, or volume you see in the application is automatically converted from its "projected CRS" value to its true "real-world" value. This means what you see on your screen—whether it's the distance between two points or the area of a roof—is a reliable, real-world measurement.
- For inputs: The feature also works in reverse. When you define a measurement, such as setting the length of a scale constraint, the value you enter is interpreted as a real-world measurement. The software then automatically accounts for the projection distortion to ensure your input is applied correctly, making sure your model is accurately scaled and geolocated.
Bear in mind that for very large projects, the distortion correction gets less accurate, as well as for images taken from a large height (for example, by plane or satellite). This advanced capability ensures that your projects are built on the most precise data possible, helping you overcome the inherent limitations of map projections.
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