Recently, we looked at GPS Precision (EPE). A few questions were also raised about plot location and I thought it might also be good to look at how accurate we could expect to locate the survey point inside the forest under a range of conditions and forest canopies.

The dataset used contains 320 survey points located throughout New Zealand. A recreational grade GPS (Garmin Map 60CSX) was used to locate the survey point in the field. At this point a survey grade Trimble ProXT GPS unit was raised to 5m and recorded 300-500 data points over a period of 20mins to 3 hours. This was then post-processed to “geo-fix” the actual field plot. Precision of the fix from the survey grade GPS was usually within 2m.  See this earlier article for more information.

Figure 1 shown below displays the horizontal distance from the intended survey point location to that actually fixed in the field using the survey grade GPS above.   This shows 99% of the plots were within 20m of their intended point, and more surprisingly 85% within 10m (with 65% within 6m !).

This shows with the correct GPS use, knowledge and experience survey points can routinely be placed within +/-20m of their intended location.


Figure 1 – Horizontal difference from intended GIS survey point to post processed fixed field plot location.

022412_0049_howaccurate2Figure 2 – Trimble ProXT in action at top of 5m pole within a typical radiate pine forest in New Zealand.


Figure 3 – Gamin Map60CSX recreational type GPS used to locate plot centers

Note we should always expect outliers when conditions for satellite reception are extreme, and an experienced field staff should realise this and use all tools given to them to locate the survey point correctly (good topographical maps, compass, altimeter etc).   This dataset was based on a nationally important permanent sample plot set, therefore staff also had access to good topographical maps and aerial photos of the plot locations to assist with placement of plot, therefore being the high end of what is achievable.

Interpine can provide trained staff or rent out high-grade GPS equipment suitable for use under forest canopy.   For more information feel free to contact our team or see here for more information.