mark-p-thomas : GIS Glossary

Coordinate

At a minimum this is a set of latitude and longitude numbers to indicate a location on Earth. May also include a timestamp if the coordinate was recorded from a GPS device.

Clipping

Removing coordinates of a GPS track from the beginning or end of a track

Cruft

The extra segments in a track created by a GPS unit recording before or after the intended activity in a different location. This may be caused by recording before the GPS has updated to the new location, or from the user not explicitly starting a fresh track at the start of the activity.

GeoJSON

Common GIS data exchange format. JSON-derived format used in applications such as Leaflet. See GeoJSON glossary.

GPX

GPS Exchange format. XML-derived & text-editable format/standard for writing GPS files. Most GPS devices output this format. See GPX glossary.

KML

Keyhole Markup Language format. XML-derived & text-editable format/standard for GIS data exchange used in applications such as Google Earth. See KML glossary.

Noise Cloud

Formed by slight inaccuracies in GPS data over time due to changes in satellite reception, this is when the GPS is stationary but records ‘moving’ points as the estimated location shifts. This tends to average over the actual coordinate.

Smoothing

Removing all suspected inaccurate coordinates between two known accurate coordinates. The resulting Track or Route becomes a straight-line interpolation between these points.

Also a consolidation of a noise cloud to a single point.

Moving inaccurate coordinates to a hypothetically accurate spot for more accurate non-linear interpolations may be able to be implemented in the future.

Splitting

Returning multiple sub-segments of a track segment based on some criteria that selects one or more coordinates to split upon. This is often done in order to allow the user to see and select the clipping and smoothing to be done.

Track

GPX track. A continuous GPS recording of movement, represented by coordinates that form a LineString or MultiLineString (for multiple track segments) in a GeoJSON file.

Track Angular Speed

The rate at which Track Rotation occurs (e.g. radians/sec).

This can be compared to expected angular speed ranges expected of a given activity to identify coordinates to fix.

For example, a rotation on a trail switchback may occur at a faster rate than it would traveling at higher speed on a bicycle. A limit of how far a rotation can occur, and how often this can recur, can also spot a noise cloud or spotty data.

Track Gap

If satellites are lost when recording a track, the track is recorded as multiple Track Segments. A Track Gap is the discontinuity between these segments and may be closed by merging all Track Segments back into a single segment.

Track Offset

If satellite reception is insufficient for accurate positioning of the GPS device, a track may not have gaps, but the accurate timestamped coordinates may be at the wrong location.

At best a reasonable path may be drawn, but it is offset and/or skewed from the actual line. At worst, the points jump around randomly based on changes in satellite communication. A key difference from a noise cloud is that this occurs as the GPS is moving, so the noise oscillates around the path traveled rather than a point.

Track Rotation

The change in track direction. Determined by comparing the direction formed between any 3 adjacent track points that in turn form 2 straight lines:

Direction 1 is a straight line formed between point i - 1 & point i. Direction 2 is a straight line formed between point i & point i + 1.

Track Speed

Speed magnitude traveled between two Track Points, derived by linear interpolation between the distances between the them and their corresponding timestamps.

This can be compared to expected speed ranges expected of a given activity in order to determine if the recording is of the wrong activity intended or determine coordinages to fix.

For example, 20 mph may be an acceptable speed for cycling, but is obviously a bad data point for a hike, regardless whether it was a GPS error or leaving the device on after the activity has ended.

Track Velocity

Track Speed combined with a direction formed by a straight-line vector joining the two Track Points.