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Because these datasets are huge, data was processed into raster DEMs as tiles.
In mosaicking them, we may encounter various problems:
- There will be some mismatch between different surveys. There will be quality variation.
- If tiles were not not produced with sufficient margins,
there can be mismatches. (I don't think this happened.)
- Some tiles were trimmed too tightly, so there are no-data gaps of a few
cells at seams. This was seen in at least one early survey.
We will check to see if it persists in mosaics.
- The corners of some tiles are not multiples of 6 feet. We are creating
our newer mosaics with bilinear resampling, so these tiles are smoothed
in the process. Here are lists of offset tiles, with the X and Y offsets
in multiples of six feet.
- When combining surveys processed in different projections (State Plane
North v. State Plane South or Universal Transverse Mercator),
data will be smoothed in reprojection.
- Someone may not have checked carefully for a matching vertical datum.
- There is a danger that gross errors, e.g. a tile being stored with the wrong projection,
might be overlooked until spotted by irate users.
- Or someone might not notice that a national park DEM is in meters rather than feet.
- Some of our newer survey were gridded at 2-foot, 3-foot or one-meter resolution
and will be resampled.
- A digital elvation model is one person's model of the real world. Different modelers
will build different models from the same data, so adjacent surveys may have
a different character.