User`s guide

R2008a
15-4
Improved Accuracy for the limitm and setpostn Functions
In previous releases, after calculating the latitude and longitude limits of the geographic
quadrangle bounding a regular data grid, function limitm arbitrarily rounded those
limits to the nearest one millionth of a degree (equivalent to about 10 cm in latitude or
equatorial longitude). Although it is small, this rounding operation in effect applied an
arbitrary shift to points on or very near the edge of the grid. The direction of the shift
and its magnitude were arbitrary because rounding can either increase or decrease a
value. In any given case, the shift depended on the specific referencing vector and the
number of columns and rows in the data grid. This behavior unnecessarily degraded the
numerical accuracy of limitm and those functions which depend on it, and it has now
been removed. For more information, see bug report 420038 on the MathWorks Web site.
In the setpostn function , an identical rounding step has been removed. Additional
changes eliminate a problem for certain input points near boundaries between grid cells
that caused row and column subscripts returned by setpostn to be off by 1. For points
near the northern and eastern edges of the data grid—but still within the grid—returned
subscript values could exceed the corresponding grid size. For more information, see bug
report 173338 on the MathWorks Web site.
Compatibility Considerations
These corrections can cause subtle changes in the behavior of other functions that work
with regular data grids referenced to latitude-longitude, for example, imbedm.
If your referencing vector contains approximations to rational numbers that do not have
an exact a 64-bit floating point representation (e.g., for cells that are 1.5 degrees wide,
refvec(1) is 0.666666...), you may still find that certain points that are extremely
close to a grid cell boundary cross into a neighboring cell just across the boundary. Such
numerical ambiguity is inevitable given how the information in a referencing vector is
encoded. Although it cannot be eliminated within setpostn, the inexactness only affects
points that fall within a few factors of eps (very much less than a millionth of a degree)
away from a given cell boundary.
New Point Location Demo Data for Tsunami Events
The Mapping Toolbox demo data in the $MATLABROOT/toolbox/map/mapdata
directory now includes a global tsunami data set in shapefile format with 'Point'
geometry. The data set comprises four files: