A tracklog is a record of where you have gone. Tracklogs are often gathered by a GPS device or a phone that collects many sample locations in memory as you travel about. These sample locations are like pins in a map: the more pins you place, the better they could show the actual route you took.

For automatic geotagging to work, each sample location should have a timestamp so that Geotagalog can guess where you were when you took each picture. If a photo was taken at a time between two location samples, Geotagalog may place the photo at a location between the two samples if its geotagging algorithm determines that interpolation would be helpful. (Interpolation is like using a string to fill the gap between pins in a map.)