| Quoting the geotiff spec:
ModelTiepointTag:
Tag = 33922 (8482.H)
Type = DOUBLE (IEEE Double precision)
N = 6*K, K = number of tiepoints
Alias: GeoreferenceTag
Owner: Intergraph
This tag stores raster->model tiepoint pairs in the order
ModelTiepointTag = (...,I,J,K, X,Y,Z...),
where (I,J,K) is the point at location (I,J) in raster space with pixel-value
K, and (X,Y,Z) is a vector in model space. In most cases the model space is
only two-dimensional, in which case both K and Z should be set to zero; this
third dimension is provided in anticipation of future support for 3D digital
elevation models and vertical coordinate systems.
A raster image may be georeferenced simply by specifying its location, size
and orientation in the model coordinate space M. This may be done by
specifying the location of three of the four bounding corner points. However,
tiepoints are only to be considered exact at the points specified; thus
defining such a set of bounding tiepoints does not imply that the model space
locations of the interior of the image may be exactly computed by a linear
interpolation of these tiepoints.
However, since the relationship between the Raster space and the model space
will often be an exact, affine transformation, this relationship can be
defined using one set of tiepoints and the "ModelPixelScaleTag", described
below, which gives the vertical and horizontal raster grid cell size,
specified in model units.
If possible, the first tiepoint placed in this tag shall be the one
establishing the location of the point (0,0) in raster space. However, if
this is not possible (for example, if (0,0) is goes to a part of model space
in which the projection is ill-defined), then there is no particular order in
which the tiepoints need be listed.
For orthorectification or mosaicking applications a large number of tiepoints
may be specified on a mesh over the raster image. However, the definition of
associated grid interpolation methods is not in the scope of the current
GeoTIFF spec.
author:
Simone Giannecchini
since:
2.3 |
