XPost: alt.comp.freeware
On Mon, 25th Mar 2024 15:06:03 -0400, Wolf Greenblatt wrote:
Exactly as I expected: Long winding discussion with no prospect of getting
to the target, eventually. Despite my initial statement I'll give you some pointers before the matter drifts even further away...
You would think every topo map program would handle geocalibrated PDFs
since every single inch of the USA has a free detailed geoPDF, most dating back to more than a hundred years & constantly forever updated ever since. https://www.usgs.gov/faqs/how-do-i-find-and-download-us-topo-and-historical-topographic-htmc-maps
GeoPdf is a rather new format and requires very sophisticated renderers. Although GeoPdf can hold both, vector and raster data, most USGS GeoPdf
combine some vector information right into the included raster. This
leads to 2 problems: Only some additional themes (like contour lines)
benefit from the free scalability of vector data. Most do not. Still,
the rendering of GeoPdf is much more compute-intense than raster image
formats, like GeoTiff, and will therefore be rather slow on display. (Especially so, when more than one GeoPdf is displayed.)
Much more of a problem, though, is the rendering of map captions, legend,
and so on right into the raster layer. As long as you only view one tile
(aka one GeoPdf file) at any given time, this is just a nuisance. But if
you want to load several tiles alongside each other, you admittedly can
make the white background of the map tile border transparent. But the
legend entries and the like will still overlap parts of the adjacent map
tiles. - Instead of the map you'll see just legend text...
GeoPdf, fortunately, is not the only map format delivered by the USGS.
IMHO, a good source for USGS map downloads is:
https://ngmdb.usgs.gov/topoview/viewer
Beside GeoPdf you can get Jpeg, GeoTiff, and KMZ. The Jpeg downloads I
tried contain no geo-reference, which makes them rather useless. Apart
from this, they also show the legend information. GeoTiff would be a
great format, but carries the legend, as well. With scripting it should
be possible to strip the border while keeping a correct geo-reference.
But this is a /lot/ of hassle.
Fortunately, the KMZ format contains borderless pure map tiles. When downloading KMZ, you get a *.zip archive, though. For newer US topo
maps, the *.zip seems to always contain 2 *.kmz files: One shows a
standard topographic map, while the other contains an aerial orthophoto.
These images are so called "ground overlays" and comprise (together
with sidecar doc.kml files) a special variety of *.kmz files.
Next question is, which GIS program supports *.kmz files. A good choice
seems to be GpxSee (www.gpxsee.org). Unfortunately, it doesn't seem to
permit seamless viewing of several *.kmz files and also (currently?)
has problems displaying GPX tracks. It has a nice map caching mode for
several online map sources, tough. And there is a repository with
configuration files for additional sources.
If a small and easy solution cannot be found, QGIS usually is the way
to look. With KML Tools plugin the ground overlays can be extracted
and converted (with the coordinates from their sidecar *.kml files)
to GeoTiff files.
IMHO, there's a better approach, though. After downloading the *.zip
with the *.kmz files, extract the latter from the *.zip. All *.kmz
files are also *.zip files (just renamed). If you can open them in a
file manger of your choice, directly, then do so. Otherwise, rename
all *.kmz files to *.zip.
Each of these USGS *.kmz/*.zip files contains a subdirectory "assets",
which you can ignore. You need the *.jpg file and the doc.kml. Extract
these in a directory of your choice and rename the doc.kml to the name
of the *.jpg. (Do /not/ change the name of the *.jpg file!)
Create different target directories for the the topo images and the
aerial images. Both *.jpg files have the same name and cannot reside
in the same directory. (If you rename a *.jpg, you need to edit the
content of the sidecar *.kml, as well. - To adjust the reference inside
it to the new name.)
If you used the KML tools export approach, you load the new GeoTiff
files into your QGIS map, afterwards. If you went the extraction path,
you don't load the *.jpg files. Instead you load the sidecar *.kml
files. You'll be asked by QGIS, which content from the *.kml you wish
to load. Ignore the point and area theme and instead load the listed
image (has a raster symbol).
Bottom line: The description is much more complicated, than executing
it. But only, if each step is really understood...
Good luck!
Bernd
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