Technische Universität Wien
Orientation and Processing of Airborne Laser Scanning data
Department of Geodesy and Geoinformation - Research Groups Photogrammetry and Remote Sensing
Improves the geo-referencing of an ALS flight block based on pairwise affine strip transformations (cf. Module LSM).
The Module GeorefApprox allows to perform the geo-referencing of a set of ALS strips so that they are finally given in one common coordinate system. Therefore, a multi-strip adjustment is carried out where the pairwise relative orientation parameters resulting from a previous step are used as observations (e.g., this can be done by applying Module LSM to each overlapping strip pair).
The basic input for Module GeorefApprox is a list of relative transformation parameters as obtained by Module LSM. Of course, all ALS strips occurring in this list must be connected to each other by relative transformations (otherwise, multi-strip adjustment will fail). By default, all those strips will be used in adjustment. However, the user may define a subset of strips by specifying the parameter stripList.
In order to define the datum, 12 parameters are chosen and kept fixed. Therefore, one of the strips is declared as reference strip. By default, the most central strip - i.e. the strip whose centre of gravity (COG) is closest to the median position of all strips' COGs - is determined and selected as reference strip. However, a user-defined reference strip may be specified using the parameter fixedStrip.
The simplest way to define the datum is to keep one complete transformation-set fixed, i.e. to set the reference strip's transformation to identity transformation. In this case, however, residual errors stemming from the mounting calibration may - when performing adjustment without any control features - cause a shift which is linearly growing for each strip with its (across-flight-direction) distance from the central fixed strip.
In order to prevent the block from this 'affine drifting', zero-shift observations are introduced for the two peripheral strips (i.e. 6 shift components are observed 0). In this case, the datum definition requires 6 further constraints: therefore, it is sufficient that 2 rotation angles, 2 scales and 2 affinities are kept fixed in the reference strip's transformation.
The parameter sigmaShift specifies the standard deviation of additional zero-shift observations (recommended value: some dm). If sigmaShift = 0, no such observations are introduced to the adjustment and the identity transformation is assigned to the reference strip in order to fix the datum. If a positve value is specified, the additional shift observations apply to the two outermost strips of the block. In case sigmaShift is negative, the fictional zero-shift observations are applied to all strips of the block.
As input datasets, the 3 files G111.las, G112.las and G113.las located in the $OPALS_ROOT/demo/ directory are used, each of which containing the point cloud of a part of an ALS strip. First, we have to import the datasets and derive grid models. Therefore, we use the modules Module Import and Module Grid. Since there are relatively much vegetated areas in this datasets, the use of grid masks is highly recommended for Module LSM. Therefore, we additionally derive sigma and excenter grid models, with the help of which we can derive grid masks using the module Module Algebra :
As result, we obtain the grid models G111.tifs, G111_sigmaZ.tif, G111_excen.tif, G112.tif, G112_sigmaZ.tif, G112_excen.tif, G113.tif, G113_sigmaZ.tif, G113_excen.tif.
Using Module Algebra, we generate the grid masks G111_mask.tif, G112_mask.tif, G113_mask.tif:
In the next step, we apply Module LSM to the pairs of neighbouring strips (using the grid masks, respectively), i.e. to pair G111/G112 and to pair G112/G113 in our case (G112 is situated between G111 and G113). We specify LSMpairs.xml as outParamFile, where all the relative transformation results are collected.
Note: Since the resulting parameters of Module LSM are used as input parameters for Module GeorefApprox, the parameter outTrafPars of Module LSM is assigned to the parameter obsTrafPars of Module GeorefApprox by default (see configuration file opals.cfg):
For further information on this, please refer to the section Parameter Mapping.
Based on the transformation parameters stored in LSMpairs.xml, the multi-strip adjustment can be carried out and the final results are written to the file geoRefApproxResult.xml:
In this case, we are not using additional shift observations and the datum is defined by keeping the central strip (G112) fixed. The results (after 2 iteration steps) are:
If we want to consider additional zero-shift observations, we have to specify the parameter sigmaShift (here, we use a sigma of 0.1m):
In this case, we obtain the following results (after 2 iteration steps):
Note: For better readability, the values extracted from the XML result file have been rounded to 5 decimal places (affine 3x3-matrix) and 3 significant digits (shift components), respectively.
1.8.17