Whenever a project is created in OneButton, it tries to calculate the altitude above ground of the sensor based on the information in the image metadata (or GPS file) and the elevation of the ground above sea level in the area of the project which is taken from 10 meter SRTM DTM grid.
GPS coordinates are based on the WGS 84 coordinate system (Earth Centred Earth Fixed or geocentric - EPSG code 4326) which uses the WGS 84 geoid and incorporates the Earth Gravitational Model (EGM96). Earth Centred Earth Fixed is a Cartesian coordinate system relative to the centre of the Earth. The offsets of this geoid from the ellipsoid provide a near sea level model and also provide a precise equation to go between the ellipsoid and raw GPS positions.
Since drones provide us geocentric WGS 84 coordinates and heights, the software must transform this information to a local projection based on the selection you make when setting up the project. Local projections are a Cartesian coordinate system, and their XY axes are approximately tangent to a geoid, the Z axis is perpendicular, and they support orthometric height measurement because of these properties.
So the drone goes from raw GPS, which is really WGS 84 Earth Centered Earth Fixed (geocentric), to WGS 84 geodetic. The software then converts the values from WGS 84 geodetic to a local projection. If the drone heights are incorrect it will cause the software to try and find an incorrect solution that will not correspond the SRTM because the SRTM is also based on the WGS84/EGM96 geoid.