Geocoding using GPS data is arguably the quickest method of adding location information to photos. The GPS data comes in the form of a tracklog file recorded by a GPS device or smartphone app. Every few seconds a record of the current time and location is added to the tracklog.
Besides location coordinates, the GPS device can record additional information like altitude, heading, or speed. When HoudahGeo matches photos to the tracklog it can copy both location coordinates and such additional information to photo metadata.
At the same time, some cameras have sensors that can provide such additional information even when not using a GPS receiver to add location coordinates to a photo.
A compass built into the camera body can, for example, provide viewing direction information. This would be more accurate than a view direction computed from the direction of travel between to locations recorded in the tracklog.
When the same information is available from two sources – the GPS tracklog and photo metadata – you are left with the choice of which to trust.
While a barometric sensor in the camera can provide altitude information, this can only be reasonably accurate when properly calibrated at every change of the weather. Conversely, a GPS device can only provide accurate altitude readings when there is a clear view of the sky.
Automatic Geocoding Using GPS Data
When you load a GPS track that provides information beyond location coordinates, HoudahGeo will present you with the GPS data geocoding settings.
Here HoudahGeo asks you which information from the GPS tracklog you want to apply to your images. You can thus choose to trust altitude information from the GPS log, but leave speed and heading values that came from the camera untouched.
HoudahGeo can geocode photos automatically: when you load photos and GPS tracklogs into a project, HoudahGeo matches these up and figures out the latitude and longitude coordinates where the photos were taken. It does so for all photos that lack location coordinates. At the same time, HoudahGeo will add supplemental information according to the choices you made in GPS data geocoding settings.
Your settings will thus apply to all images you add to your HoudahGeo project. This may not always be desirable. Your camera may, for example, have provided altitude values for some, but not all images. In that situation, you will want to turn off automatic geocoding. That is, you will want to uncheck the box for geocode automatically.
Instead, you can manually trigger geocoding from GPS data:
- Select the images you want to geocode using GPS data. E.g. images lacking altitude information
- Select Process > Geocode Using GPS Data… from the menu
- Tick the Selected Images Only checkbox
- Select the image properties you want geocoding to update
- Click OK to process the selected batch of images
- Repeat with a different batch of images. E.g. images having altitude information
With the same procedure, you can also force geocoding for images that were skipped by automatic geocoding because they already came with location coordinates.
Tip: Click the table headers in the list of images to sort photos. When you sort by altitude, images that lack an altitude value appear at the top of the list. Click the first one and then shift-click the last one to select them all for geocoding.
Update Photo Metadata
Once you have geocoded your photos, you will want to write the new location information to EXIF/XMP metadata tags within the image files. I.e. you will actually geotag your photos.
Select Output > EXIF/XMP Export… from the menu. Before committing the updated location information to the actual image files, you can choose which metadata properties HoudahGeo will actually write out.
Here you can, for example, prevent HoudahGeo from altering the viewing direction information in the image metadata. This can be useful in setups where you prefer to trust the information recorded by the camera’s sensors. With the option to write viewing direction metadata disabled, you can protect these values from accidental edits you may have made during geocoding in HoudahGeo.
Accuracy of Elevation Data Sources
When you add altitude information to your photos, you need to decide which source of information to trust: 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.