Hi gang,

I see that HoudahGeo will interpolate from a GPS track.  I have a question about that process.  Recently I was on a cruise, and had an app on my iPhone generate the track while the ship was sailing.  Occasionally, the app would lose track of the GPS signal.  Sometimes this gap would be for many hours.  However, the ship was generally sailing in a straight line during these times.  Is it possible for HoudahGeo to interpolate in a straight line between two points that are separated by many miles and many hours?  What would be the appropriate settings to make this work?


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Sorry for the delayed reply. I thought I had replied, but the forum proves me wrong.

The automatic geocoding process lets you set 3 values to customize how photos are matches to track points:

- Extension limit: Number of minutes by which to extend the search for tracks matching a given photo. A photo is considered to match up with a track if it falls within the track's time span or within the speci"ed number of minutes before the beginning or after the end of that time span. The default value is 5 minutes.

- Merging limit: GPS devices may start new tracks when turned on or after losing GPS signal. Strictly speaking we cannot geocode a photo that falls between two tracks. HoudahGeo however decides to merge two tracks if the time elapsed between the first track's end and the other track's beginning is less than the specified number of minutes. The default value is 30 minutes.

- Matching limit: HoudahGeo determines the location where a photo was taken by matching it up with two track points: one preceding and one following the time the photo was taken. If the closest point's timestamp is more than the specified number of minutes before or after the photo was taken, HoudahGeo considers there is no match and does not geocode the photo. The default value of 0 disables this check.

With these options you should be able to adapt the process to work with the track recordings you have.

There is however one caveat: HoudahGeo performs linear interpolation between latitude/longitude values. Over short distances this gives correct results. With longer distances between two data points you might start to notice inaccuracies.

Pierre Bernard
Houdah Software s. à r. l.

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