Information regarding image geometry and location is contained in the image's data descriptor record (DDR). Functions in LAS 7.1 utilize and update this information while functions in the Geometric Manipulation Package often determine image geometry and originate this data location information.
Tracking location and projection related information in the DDR helps to automate the mosaicking, combining, or overlaying of images in like projections. It assists in the verification of the registration process and allows the user the freedom to select any pixel at some point in the processing of an image and determine that pixel's projection coordinates. Once projection coordinates have been found, latitude and longitude may be calculated using the appropriate function.
The coordinate system of the projection plane is a cartesian coordinate system--the projection coordinate is designated by its distance from two perpendicular axes. The point at which these axes cross is the projection origin. Terms commonly used to name these coordinates are: projection coordinates, northings and eastings, (U,V) and (X,Y).
Image coordinates (line, sample) are also in a cartesian coordinate system. In many cases, the axes of the projection coordinate system and the axes of the image coordinate system are parallel. If this is so, translating and scaling is all that is needed to convert between the two systems. Projection coordinates may be converted to image coordinates when one common point in the two coordinate systems and the pixel size in projection units is known.
In the following figure, if the upper-left image coordinate and upper-left projection coordinate is known, the projection coordinate of a window beginning at image coordinate (1000,1000) may be found as follows:
proj_y = upper_left_proj_y - ((line - 1) * pixel_size_y)
proj_x = upper_left_proj_x + ((sample - 1) * pixel_size_x)
and in this example:
proj_y = 77900.0 - ((1000 - 1) * 25.0) = 52925.0
proj_x = -83575 + ((1000 - 1) * 25.0) = -58600.0
In most cases the projection coordinate system is parallel to the image coordinate system. It is possible, however, to have a projection coordinate system which is rotated from the image coordinate system, as shown in the following figure. Converting between the two systems requires rotation in addition to translating and scaling. An example of this condition is processed TM data (TM-P) as read by ccttipsp. The DDR update routines used by all LAS modules handle this condition.
A list and brief description of each projection related field stored in the DDR follows.
Projection Code Numeric code of projection system used. Refer
to Appendix E for a list of supported projections.
Zone Code Numeric code of projection zone (UTM or State Plane
Coordinate System). Appendix E contains tables
listing zones and their corresponding area of
coverage.
Datum Code Numeric code for datum used. Refer to Appendix E
for a list of datums supported.
Projection Units This is an ASCII field containing the unit of
measure for the projection system. The Projection
Transformation Package supports "degrees", "seconds",
"radians", "feet", "meters", and "dms" (which is a packed
degrees/minutes/seconds format). These unit names are
standard. It is also standard that they be in lower case
but any case will work. Since this field is ASCII, units
other than these types may be used. However, the Projection
Transformation Package will work only with the units it
supports.
Projection Parameters An array of 15 projection parameters containing
parameters specific to a given projection. A table
describing the contents of this array for a given
projection is given in Appendix E.
Upper-Left Corner The projection coordinate at pixel (1,1).
Lower-Left Corner The projection coordinate at pixel (NL,1),
where NL is the number of image lines.
Upper-Right Corner The projection coordinate at pixel (1,NS),
where NS is the number of image samples.
Lower-Right Corner The projection coordinate at pixel (NL,NS),
where NL is the number of image lines and NS is the
number of image samples.
Projection Distance The amount of projection distance in X and
in Y that one image pixel covers (also referred to as
pixel size).
Refer to the LAS Programmer's Guide for a description of the DDR and the manner in which each LAS module updates the DDR.