| Re: Map Projections and Contour Plots. [message #15812] |
Fri, 11 June 1999 00:00 |
Martin Schultz
Messages: 515
Registered: August 1997
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Senior Member |
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Grady Daub wrote:
>
> I have data that corresponds to latitude and longitude.
> example. data=40 , lat=20, lon=20
>
> How would I...
>
> ...make the correct arrays expected by CONTOUR?
> /IRREGULAR and TRIANGULATION work, but, I think they're
> causing problems.
Take a look at the TRIANGULATE procedure (it's been a while since I last
used it, bu it worked fine)
>
> ...smooth the contours?
>
> The documentation says something about MIN_CURVE_SURF, but, it > seems to need the same format array as CONTOUR.
Here you may want to consider TRIGRID which you can call after
TRIANGULATE.
>
> ...plot the contours correctly over a map projection?
>
Take a look at David Fanning's web site at http://www.dfanning.com/
In general you need to MAP_SET your projection first, then
overlay your CONTOURs with the /OVERPLOT option.
> When I try to CONTOUR over a map projection, I get some lines
> across the plot. The lines look like the contour tried to plot > from, say, longitude 180 to -180.
LONs and LATs have to be monotonically increasing for a contour plot.
Default is -180 to 180 for a global map, but if you want to center your
map say on the Pacific, you need to "convert" your longitude coordinates
to e.g. 0 to 360. You can try my convert_lon program (attached) for
this. It is called as
convert_lon,lon [,/ATLANTIC] [,/PACIFIC]
and makes sure you get longitudes modulo 360.
As for MAP_SET, you will need to set the P0Lon parameter so that it lies
within your LIMIT range.
Finally (to complete the most prominent issues with contours on maps),
if you plan to plot filled contours, there are occasions when you need
the /CELL_FILL keyword *AND* CONTOUR will use the first fill color for
values ABOVE your first threshold, i.e. you should add a very low number
to your C_LEVEL specification if you have one.
Hope, this helps,
Martin.
>
> So far, I'm only able to put a map ONTO a CONTOUR, not the other way
> around. This doesn't help me when I want to use, say, /MOLLWEIDE.
>
> -Grady Daub
> GFDI
>
> To reply be email, please, remove "ZOOKS" and "MMER".
--
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Martin Schultz, DEAS, Harvard University, 29 Oxford St., Pierce 109,
Cambridge, MA 02138 phone (617) 496 8318 fax (617) 495 4551
e-mail mgs@io.harvard.edu web http://www-as/people/staff/mgs/
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