Globe
Pan and Zoom
This part describes the way MapClient implements moving around and zooming in
and out of maps. The maps used are quite large, so they show a lot of detail.
This is also are very fine example of advanced MapServer technologies: tiling,
resampling, integration with Grass GIS, etc. It is really a marvel to see how
quickly and effortlessly Mapserver manages all these big data sets. They are
all from the public domain and can be downloaded for free, although some
processing is required to get them into MapServer. I did this using the public
domain
GRASS GIS,
and Frank Warmerdam's excellent
GDAL
utilities, which can manage every raster format known to man.
All raster maps have a grid spacing of 30 seconds (1/120 degree).
Consequently, every map is 43200 by 21600 cells.
At the
equator one grid cell is about one km; progressing north or south cells get
smaller.
Uncompressed, all maps are about 1G, compressed they measure from 30M
to 400M, depending on the variability of the cells.
The first example is the "Global Land One-km Base Elevation (Globe)" map
from the
Globe Task team.
Elevation ranges from -407 to 8752 meters, and has been colored in blue, green,
yellow, brown and grey ramps.
When zooming in on this map,e.g. in Central Europe
,
different levels of detail become visible (e.g. the Alps and the Rhine in
the last ones), until at last the km grid shows up
.
Panning works as expected, like here from Minneapolis to Mexico City
.
Independently from the geographical extent, the browser window can be made
larger or smaller, like in this picture from the Himalays
.