Centimeter positioning with GPS in forest

Thomas Hörnlund at Svartberget has done method development and tests regarding GPS:es and the difficulties to get good and accurate positions within a forest stand.
The questions originates when a researchers wanted to get the field equipment marked on a map. We knew that the installations was located five meters from each other but they did not end up five meter from each other on the produced map, says Thomas. Even though we had a pretty good instrument, Trimble Geoexplorer6000 the positions weren’t where we had expected. On open fields, with a two meter antenna the GPS received 8000 fix points within a diameter of a fist, five cm radius cm, which is what you expect from this device.
In forests with the same equipment it took tree days too receive 13 000 fix points and they had as much as five m radius of scattered points. This scattered chart was possible to improve when the antenna height was adjusted resulting in a precision around an A4 paper size.
Equiptment setup in field. Photo by Thomas Hörnlund.
Equiptment setup in field. Photo by Thomas Hörnlund.
Thomas says that these results will influence the routines used for GPS point measurements at Svartberget.
We won’t use the average values of Garmin anymore at least, because waiting for the Garmin GPS to stabilize isn’t worth since it is off anyway and remember that the precision of a 5000kr Garmin in open field is not better than a radius of 6m.
 In the longer perspective this is of high importance for all types of GPS related work and especially the accuracy in precision works e.g. forestry and farming in relation to riparian zones, ancient monuments and spot fertilization in relation to specific crop-needs and soil properties.  
Thomas Hörnlund, from Svartberget presented this study "Erfarenheter från GPS-mätning i skog med cm-upplösning" at Ljungberg Laboratory in Umeå, staff at Ljungberg laboratory has also assisted Röbäcksdalen during the initiation of UAW flights in SITES Spectral.

The presentation is available in Swedish here. Last link under Previous Seminars, December 8, 2016.

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