In the app Arboreal Forest, a local coordinate system is created that is based on AR. It uses the camera and internal sensors in the phone and not the GPS.
Arboreal Forest saves the position of each tree measured (Table 1). The local coordinates could be used to match trees with remote data such as airborne Lidar, drone images or aerial images.
To be able to do this well, we need to rotate the local coordinate system. On the iPhone, you can automatically orient the local coordinate system to the north, but it has been shown that it rotates during the measuring session. The phone uses a magnetometer to sense the earth’s magnetic field and it is constantly updated.
During the autumn of 2021, we oriented the local coordinate system to the north after the measuring session was completed. But it turned out that it did not always succeed. Sometimes the local coordinate system could point in the wrong direction.
We have come to the conclusion that it is because we sometimes get an incorrect estimate from the magnetometer about which direction the north is. It happens more often when you are in a magnetic field such as an office or when you have your phone near another phone. Feel free to try it yourself by opening the compass app on your phone and see how well it points to the north.
To be able to draw the user’s attention to problems with the compass, we added a north arrow in the Arboreal Forest app (version 2.25) which is visible before you place the center of the sample surface (Figure 1). Then you can see if it is pointing in the right direction. The arrow points to geographical north and not magnetic north.
If the north arrow points incorrectly and you want to orient the sample plot correctly, make sure you move away from the magnetic field you are affected by and walk around so that the compass in the phone starts to sense the earth’s magnetic field.
In Arboreal Forest you can do a line transect where you measure trees within a certain distance from a line. Each time the user crosses the line, we save GPS coordinates and local coordinates.
Afterwards it is possible to use this data to get a compass bearing on the line from the GPS coordinates. It provides an estimate of the compass bearing that is independent of the phone’s built-in magnetometer. See example in Figure 2 which shows GPS coordinates when the user crosses the line.
For most users of Arboreal Forest, it does not matter if the test plot is oriented in the right direction. The area measurement is not affected by problem with the magnetometer.
If you are going to use the data to match with individual trees from aerial images, drone images or lidar scans, you should check that the north arrow is correct before you start measuring.