Horizontal shift after a coordinate system transformation is applied - PIX4Dmapper

This article explains the horizontal shift a after a coordinate system is applied in PIX4Dmapper.

Error

When processing a project with accurate geotags in WGS 84 coordinate system without using GCPs or when the GCPs coordinate system does not match the output coordinate system, a horizontal shift may occur between the reconstructed scene and ground truth/verification data.

Description

If a horizontal coordinate system transformation is required to project the image or the GCPs coordinate system to the output coordinate system, an offset could be introduced between the model and check points or GCPs, when these are added.

Pix4D acknowledges the requirement of a more accurate than the current coordinate system transformation for projects referring to the Swiss coordinate system (CH1903/LV95 and CH1903+/LV95) or to the British coordinate system (OSGB36). However, it is possible that there are more coordinate systems requiring grid corrections.

Cause

Horizontal grid corrections are not currently supported. The coordinate system transformations in PIX4Dmapper are based on the GDAL library. In some cases, a constant offset is applied while in reality, a more accurate transformation needs to be applied for some pairs of coordinate systems.

Solution

To ensure accurate results there are two workarounds:

1. Transform the image geotags to the output coordinate system before importing them. There are several online transformation tools, such as the REFRAME to transform CH1903/LV03 coordinates to CH1903+/LV95 and Ordnance Survey's coordinate transformation tool to transform OSGB36 coordinates to WGS 84. Ensure that the correct coordinate system is selected for the image geolocation.
2. Introduce GCPs measured in the output coordinate system of the project: How to include GCPs in the project. PIX4Dmapper assigns a higher weight to the GCPs and they will correct for any errors introduced due to the incorrect transformation.