The new GPS track filters in GPSLog Labs are powerful, but if you had to configure them by hand for every log that you uploaded, they wouldn't be much use.

Thankfully, the real power of the filters is that they can be attached to tags, and then when that tag is assigned to a log the filters will be copied across automatically.

This means you can configure a set of filters for all your logs tagged with Run, and you can have a different set of filters for Bike Ride. For example, you may want to tweak the sample window size on the Median filter as the average speed will vary less when running than when bike riding so the filter can be a bit more aggressive.

The process for adding filters to a tag is the same as for a log, the difference is that you can't see what is being matched by the filter, so I suggest you get the filter working on a log then copy the details across to the tag so it is automatically applied to other logs.

Note that the filters are only automatically copied across to the log when the tag is assigned to the log and you have not modified the log filters. You can click the Reset button on the log filters page to restore it to the defaults for the tag.

As always, let me know if you have any difficulties or suggestions, and I hope you enjoy this new functionality.

In addition to the automatic discard filters, GPSLog Labs has 2 "manual" filters you can use to clean bad data out of GPS tracks that the automatic filters can't handle.

Manual Filter

This filter lets you discard logged data between two times. E.g. if you spent some time inside a building and got spurious signals as a result, you can "zero out" the data between when you entered the building and when you left.

Select the time range by highlighting the portion of the log on the graph, or adjusting the slider, or keying the times into the text boxes:

The data between those two times will be discarded and GPSLog Labs will interpolate the log between the remaining points:

You can use as many manual discard filters as necessary to clean up your log.

Position Filter

This filter lets you discard logged data in a specified area. E.g. if you have travelled between buildings or through an underpass where you get bad signals you can zero all points within that area.

Select the area to remove by moving the marker on the map and adjusting the radius:

The data between within the selected area will be discarded and GPSLog Labs will interpolate the log along a straight line between the remaining points. This won't be perfect, but will be better than the original noisy data. Like Places, the discard filter is "sticky" so that if the track leaves the radius briefly (less than 60 seconds) it will discard everything between the first point inside the radius and the last. This should mean you don't have to make the radius too large and will still be able to capture all the bad points.

You can use as many radius discard filters as necessary to clean up your log.

GPSLog Labs currently has 2 "discard" filters that run automatically to clean bad data out of GPS tracks.

Speed Filter

This filter is the easiest to understand: It will discard any points where the speed is above a certain threshold.

It has two parameters, the cutoff speed (e.g. 20 km/h for running) and the number of points to discard either side of the "bad" point.

After the points have been discarded, the log will be interpolated between the remaining points, hopefully giving you a more accurate trace of your activity.

Static Navigation

The static navigation filter will only be useful if you have a GPS logger with a chipset that includes this "feature" and you can't disable it (e.g. a logger using the SiRF Star III chipset such as the AMOD AGL 3080.) More details on static navigation can be found by reading http://www.etenblog.com/2007/11/20/static-navigation-explained/ and http://www.gpspassion.com/forumsen/topic.asp?TOPIC_ID=25575.

Basically, in this mode (which is optimized for driving, not walking or running), when your speed is below 5km/h the logger will "clamp" your position on the assumption it's getting some kind of signal "drift". When it decides you really have moved the logged position will "jump" giving you a log that looks something like the following:

Applying a simple median filter to this data doesn't really help (see the detail below) as there are still lots of samples at the same location:

The static navigation filter will detect these points and remove not just the point, but as many of the "clamped" points as it can, giving a much better result:

(Note that this still needs a median filter to get the best results.)

The next post will describe the "manual" discard filters.

GPSLog Labs now has a powerful and customizable filtering feature to clean up errors in your logs caused by bad GPS signals.

There is a new menu option on the log detail page that will let you edit the filters applied to that log:

The following graph comes from a log recorded while running and shows the kind of noisy data that is often present. The incorrect speeds mess up the maximum speed, average speed and distance stats and often correspond to a bad trace on the map too.

Discard filters

The first filters you can choose from will discard invalid points from your log and then interpolate the speed and course between the remaining points.

After applying those filters, the speed vs time graph looks a lot better (discarded points are colour-coded to match up with the chosen filters):

Post filters

The second filtering stage applies an average function to the newly cleaned up data. Smoothing the speed graph will give you a more reliable average and maximum speed statistic and also improve the distance and time split data, but won't clean up the trace on the map or affect the total distance.

The original speeds are shown on the graph below as a light blue line, and after applying a median filter, the thick blue line has been smoothed (no discard filters were applied for this graph).

This graph shows the final results when both discard and post filters are applied:

Existing logs

The new filters will only take affect for new uploads, existing logs will retain the data from the old filter that GPSLog Labs used to apply until they are reprocessed.

Also, the discarded points from the old filter will no longer be shown on the Speed vs Time graph on the log detail other graphs tab. This information is only available now for logs using the new filter system from the Filters menu item.

Try it out

I suggest using a simple Speed Median filter initially, as that will clean out a lot of noise, setting the window time parameter to a larger value will smooth the speed more. I'll be describing the other filters in future posts.

As always, let me know if you have any difficulties or suggestions, and I hope this new feature helps you get more out of the site and your GPS logger!