> I have managed to figure out exactly how the URL for the
> Australian Geoscience map server works.
>
> Here is a breakdown.
>
> I picked on a town called Robertson that is not that from from
> me. I found it's location on mapblast for comparison.
>
> 150.59324,-34.59181
>
> To get the Geoscience server to produce a map with Robetson
> exactly in the middle I had to pass it two cordinates.
> For the longitude I had subtract .8 degrees and add .8 degres
> For the latitude I had to add .6 degrees and subtract .6
> degrees (If I try different values here the map isn't centered)
> So the cordinates came out to...
> 149.79324 -33.99181 151.39324 -35.19181
Note .8*cos(lat) gives ~ 0.6 for S35.6 (equal area map).
0.8 is the Geoscience scale factor in this case; is there only one zoom
setting? 1:50000 maps are useful for driving.
1.2 deg lat from south to north edge * 111.12km/deg = 133.34km for each side of
the map; does this seem correct?
> There are a whole stack of &layer options that add detail to
> the map and then there is the bit that specifies the size of
> the map.
> &mapsize=1000+800. (I know I'll have to resize it to 1280 x
> 1024)
Use pnmscale or similar from the netpbm tools.
> From this I figured out that 204 Pixels = 36km or One
> pixel = 176.47m. I didn't have much success trying to zoom in
> or out but this scale seems to be the most usefull and it will
> certainly do to start with.
>
> Any ideas how to figure out that the relative scale of the map
> is?
from my GRASS GIS d.out.gpsdrive script:
EXTENT=`g.region -e | grep 'north-south extent' | cut -f2 -d':' | awk '{print
$1}'`
M_PER_PIXEL=`echo $EXTENT $FRAME_HEIGHT | awk '{printf("%f", ($1 * 1852*60.) /
$2)}'`
(you've already figured this out, but from my above calculations, 133.34km /
800 pixels = 167m ... we are close)
SCALE=`echo $M_PER_PIXEL | awk '{printf("%d", 0.5 + ($1 * 2817.947378) )}'`
what that means is multiply your meters/pixel number by 2817.947378 (hard coded
in gpsdrive) to get the scale. As best as I could figure out that number comes
from someone's particular pixels/monitor width. As monitors are never at the
same resolution or size, scale will always be a bit of a general guide instead
of "correct", but there's not too much we can do about that.
the %d, +0.5 is to overcome truncation & do rounding in awk.
best of luck,
Hamish
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