Why are my survey points shifted?

It happens that when you collect data and then checking it in the office on yourdesktop using QGIS, the points are misplaced and shifted. This blog post explains the possible root cause of this issue?(Read time 5 min)

This article follows the help document about the projections and coordinate reference system handling in QGIS.

The accuracy of the captured points is affected by two main factors- GPS receiver and accuracy- Selection of coordinate reference systems used in the project

Lets take a closer look at both problems in context of field surveys.

GPS accuracy

GPS accuracy depends on the quality of your GPS receiver and the number of visible GPSsatellites your receiver can use at the moment of capturing the point.

When you load the map in Input app, the bottom bar contains the GPS marker with a coloured dot.The dot can have the following three colours: - green: The actual accuracy is below the threshold - orange: The actual accuracy is above the threshold - red: The GPS is unavailable

For different use cases, acceptable accuracy is different. So the threshold for thecolour scheme could be adjusted in the Input app settings. Always adjust the settingsbased on your project needs and check the GPS accuracy when capturing data.

The GPS receiver itself can be either improved by usage of device with a better internalhardware or usage of powerful external GPS receivers If you want to use the externalGPS receiver, read the following help articlefor setup.

Projection problems

First of all, there could be multiple problems with setup of coordinate reference systems andtransformations. To solve it, you need to load the project in QGIS, usingMergin Plugin and check for various situationsas described in this help article.

Input app checks for missing datum shift files or other transformation problems and notifyyou once at the project load by following message.

To be able to use the datum grid shift in your mobile app:

• Copy the proj folder from your QGIS working directory (see help page for different platforms)
• Paste the folder to your survey project downloaded fromt he Mergin server
• Sync the changes

For our example with the British national grid on Windows, the grid file is calledOSTN15_NTv2_OSGBtoETRS.gsb and if the Mergin project is inC:\users\qgis\myproject then it should be copied toC:\users\qgis\myproject\proj directory and synchronised by the Mergin service to the Input app.

Notes:- This is one-off process. Input app will transfer the grid shift datum to its own working directory on your mobile device.- There might be more than one datum grid shift in your QGIS working directory. Use the appropriate one for your survey project.

You can find a short summary of this article on the Input app help pages