|Objective of field work on MARV4/2
In photogrammetric STRS inventory our measurements and model estimates are prone to both systematic and random errors. It is possible to reduce some of this inaccuracy by calibration. For that we need measurements in the field that are one-to-one linkable with the STRS-trees. Also, the use of STRS leads to an unavoidable underestimation of the tree population, since the small trees remain unseen. We wish to know what this population is, and if the omission error-rate (process) is affected by the properties of the upper canopy. STRS can go wrong in that it produces false trees - commission errors. These are even more disturbing than the missed trees. We can detect the ghost trees only by visiting the field.
Fig. 1. An example of STRS measurements on a map. The trees have symbols that reflect the species and the photogrammetric height. Treetop positions are reasonable estimates for the stem positions - at least for the non-slant, vertical stems that have a peaked crown. The map is not complete: Some trees may have been missed and some trees are clearly in a wrong position. The HSV-colored bitmap represents the DTM in 1-m cells.
Fig 2. The tree map (STRS measurements) can be processed into tree labels, which have the following information: serial number, plot ID, distance from the plot center, azimuth observed by a compass from the plot center, tree ID, map of neighbors with the ID and excepted azimuth, species and height.
In the field, inside a 11.28-m circular plot
Fig. 3. Once the photo-trees (PA) have been identified in the field, it is possible to use them for the positioning of other targets (P0). Interpoint (intretree) azimuths and/or distances are needed for that. In the process, which is combined triangulation and trilateration - standard method in geodesy - the unknown point P0 is located. An error estimate is obtained also, if redundant (extra, 3 or more) observations were done. Intertree azimuths are observed with a precision compass ('bussoli') and intertree distances using a laser rangefinder or a metal tape in dense vegetation.