Nine forest managers or practitioners participated in the workshop in Montafon (Austria), which aimed at validating the forest maps derived from airborne laser-scanning (ALS) data using object based image analysis (OBIA) in the field. The forest maps were assessed in respect to their information content, usability in the field and as planning basis for forest management strategies.
Surface models and a forest mask derived from ALS data acquired in the year 2011 were used for creating the forest maps. The information maps reflect height information of forest canopy on a lower tree crown level (TCL) and a higher forest stand level (FSL), which were derived by segmentations of the normalized digital surface model (nDSM, GSD: 0.5 m) and/or derivatives. A classification scheme was developed to classify the forest stand height by the 90th percentile of height above ground into height layers: lower than 1.3 m or not forest area, 1.3 to 6 m, 6 to 11 m, 11 to 22 m, 22 to 29 m, 29 to 33 m and higher than 33 m. These forest height layers correspond to forest developmental stages, such as youth, pole stand or old growth. For field validation and verification various maps were produced with different zooming level, namely with map scale of 1:1000, 1:1500 and 1:5000, whereas the latter reflects map scale of the final target product. In order to facilitate orientation in the field, overview maps with orthofoto and hillshades of the surface model were prepared.
The Expert based field validation was carried out at two reference locations (Appendix X). One Reference location was characterized as spruce forest (Piceetum) at ca. 1050 m above sea level (asl). The second reference location was characterized as mixed forest composed of Picea abies, Abies alba, and Fagus sylvatica (Abieti-Fagetum) at ca. 860 m asl on a south-facing slope. On these locations the participants compared the height patches on the different maps with the situation on site, in order to assess if the delineation of the homogenous patches and the height classification was correct.
ALS (Airborne Laser Scanning) data provide very useful additional information for supporting decisions in forest management planning. The participants were impressed by the degree of detail and precision. The delineated height patches of the tree crown level (TCL) appeared on the first sight as too small. In the discussion it turned out that two separate maps with the different levels would suit the practical needs best.
The applied method of image segmentation of nDSM in an OBIA environment is promising due to the reproducibility, transparency and repeatability of the application on future ALS campaigns. The classification scheme was regarded as reasonable and very useful in the field, as well as for management planning. If the slope map is included in the segmentation process for the FSL the resulting objects also reflect specific morphological characteristics of the mountainous terrain. Forest roads e.g. can then be recognized in the pattern of objects. Thus the slope derivative of the digital terrain model should be included in the segmentation process of the FSL. The map scale of 1:5000 shows sufficient details for forest management planning and field work.
Outlook – next steps
Calculation of information maps TCL and FSL for the whole forest area of the communal administration Stand MontafonForstfonds (ca. 89 km2) and the whole area of Montafon (ca. 160 km2).
Implementation of the forest maps in the work routine
The minutes of the workshop are available to logged-in users.