Troubleshooting
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      Wrong elevation on the DSM - PIX4Dfields

      Introduction

      Accurate elevation values are essential for generating a digital surface model (DSM) from drone imagery and for a wide range of agricultural tasks. However, discrepancies in the DSM elevations in PIX4Dfields and the expected values may arise, even when using a Real-Time Kinematic (RTK) system to improve positional precision. 

      The primary issue stems from the fact that many drones record their altitude as ellipsoidal heights, and PIX4Dfields does not automatically convert ellipsoidal heights to orthometric heights (above sea level). PIX4Dfields displays the raw height values provided by the drone. As a result, the displayed elevations can appear significantly incorrect when compared to above sea level heights.


      Ellipsoidal Heights:       These are elevations measured relative to a mathematical model of the Earth (the ellipsoid).
      Orthometric Heights: This references the geoid, an approximation of the mean sea level.

      How to solve it? 

      PIX4Dfields does not allow changing between ellipsoidal or orthometric heights.

      To address this issue effectively, follow these recommended steps:

      Determine the required altitude offset

      If the discrepancy is primarily due to a mismatch between ellipsoidal and orthometric heights, identify the approximate height for the project area. This value often represents the shift needed to convert from ellipsoidal to orthometric heights (or vice versa).
      Check local geoid models or official survey data for more accurate offset values.

      For instance, if PIX4Dfields indicates an elevation of 100 meters, but the actual expected altitude for that location is 50 meters, the resulting discrepancy of 50 meters needs to be accounted for in QGIS.

      Use the QGIS Raster Calculator

      1. Export Surface model from PIX4Dfields. 
        1. Go to Export, Layers.
        2. Choose .data.tif, Embedded overviews and click Export:
          ExcporDSM
      2. Load the "Surface model.data.tiff" into QGIS.
      3. Open the Raster Calculator (Raster > Raster Calculator). Please watch the following video.
      4. Apply a simple arithmetic correction by subtracting (or adding) the known offset.
        For example, if PIX4Dfields elevation is approximately 50 meters higher than the expected one, add a formula like: "Surface model.data@1" - 50 

        RasterCAlc

      5. Save the corrected DSM. 
      6. Import it back to PIX4Dfields if needed:
        1. Click Import inside the project.
          ImportImages-1
        2. Click GeoTiff under Other maps.