Some GPS cameras use a barometer to determine the current altitude. These cameras can write altitude information to photos even when the GPS feature is switched off. This is often seen in Panasonic cameras.
The camera creates photos that have altitude information, but lack latitude and longitude coordinates. This comes as a bit of a surprise to HoudahGeo and requires a few additional clicks on your part.
HoudahGeo wants to automatically geocode photos: when you load photos and GPS tracklogs into a project, HoudahGeo will automatically match these up and figure out the latitude and longitude coordinates where the photos were taken. Many GPS trackers also provide altitude information. HoudahGeo will add that too to the photos.
HoudahGeo also has to protect information that already exists in photo metadata. When you load a photo that has GPS coordinates, HoudahGeo does not know where that information originated from. It may be more accurate than what your GPS track logger provides. It may be location information that you put work into looking up manually. Thus when you hand HoudahGeo photo that has latitude, longitude, or altitude information, geocoding from a GPS tracklog will not kick in automatically.
This creates a small problem: HoudahGeo will not automatically compute latitude and longitude for photos that come with altitude information.
That problem is solved easily. Select Process > Geocode using GPS Data… from the menu. Decide if you want to process all photos in the project or only the selected ones. You can leave the other settings untouched.
HoudahGeo will now compute latitude, longitude, and altitude for the photos in your project. Depending on what information is available in the GPS tracklog, it may also add speed and direction of travel.
The altitude information recorded by the camera is now gone from the HoudahGeo project. The image files, however, have not yet been modified.
Later, when you export the new geotags to EXIF/XMP metadata within the image files, you will need to decide if you want to overwrite the altitude values with those computed from the tracklog. Check the option to write altitude information to write the computed values to the file. Uncheck the option to keep the old values and have HoudahGeo only add location information and other metadata.
Accuracy of Elevation Data Sources
You need to decide which altitude information to write to your photos. The value recorded by your GPS device or your camera, or a value obtained from a lookup service.
GPS track loggers can record accurate altitude information when they have a clear view of at least 4 GPS satellites. I.e. there should be no tall buildings, deep valleys, or overhead cover.
Barometers can give a good indication of altitude when frequently calibrated. These devices determine altitude from air pressure. This varies with temperature and weather. Calibrate your GPS or altimeter watch at the start of the day and you can hope for an accurate reading later in the day provided the weather remained stable.
Altitude lookup services can give you the altitude at a specified location. These services provided generally accurate information. They, however, don’t have an exact measurement of the elevation at each point of the globe. Instead, they rely on models of terrain. You are likely to see the same value for two close-by locations as these are part of a single area.