| Re: map_grid limitations [message #30172] |
Thu, 11 April 2002 11:13  |
Ken Mankoff
Messages: 158
Registered: February 2000
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Senior Member |
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I think your suggestion is to translate all my datasets to a -180 to
180 East grid. This is an option I am considering...
But all the globes in the office and around the lab are made on a 0 to
360 west grid, and it is nice to have the numbers on the screen agree
with those on the globe.
I do not want to try to change the coordinate systems of the planets
to meet IDLs requirements. Nor do I want my software to disagree with
most other peoples, the USGS, etc.
-k.
On Thu, 11 Apr 2002, Luis Alonso wrote:
> Hi!
> Why don't yo try, the same way you change the labels, you can change the
> header of you map in which the coordinates are stored. Just sustract 180 to
> the longitude.
> In case it is stored in a header file where it gives the firts pixel
> coordinate (such as ENVI format) just sustract 180 in the corresponding line
> (x_start for the ENVI case)
> In case you hace a matching array (lat_coord for example) storing the
> coordinates, just do:
> lat_coord = lat_coord -180
>
> Hope it helps,
> Luis
>
>
>
> "Ken Mankoff" <mankoff@I.HATE.SPAM.cs.colorado.edu> escribi� en el mensaje
>
>> Hi,
>>
>> I am interested in using IDLs mapping procedures on Mars, which has a
>> fair number of datasets on the 0 to 360 *west* longitude system as
>> opposed to the -180 to 180 *east* system.
>>
>> I posted this question a week or two ago, but got no responses. I
>> don't think I stated the problem terribly clearly in the original
>> post, so I am trying again. Also, IDL wrote me and said that:
>>
>>> Some national labs have changed these routines (which are *.pro
>>> code) to work with the Moon, so check the newsgroup.
>>
>> After an extensive web-search, I found a "mapl_set" procedure
>> mentioned here:
>> http://www.ias.fr/cdp/Futur/perso/biblio2.html
>> But there is no link and no code, only a description!
>>
>> ------------------------------------------------------------ ----------
>> Here is a demonstration of the problem:
>> A simple map in IDL is created like this:
>> IDL> map_set, 0, 0, /cylindrical
>> IDL> map_grid, /label
>>
>> You will notice that the longitudes increase to the east, and are on a
>> -180 to 180 degree grid. I can get the appearance of longitudes
>> increasing to the west and on a 0 to 360 grid by these commands:
>> IDL> map_set, 0, 0, /cylindrical
>> IDL> map_grid, lons=[0,45,90,135,180,-135,-90,-45], $
>> lonnames=[0,315,270,225,180,135,90,45], /label
>>
>> Everything appears fine. But the problem is that it is only the labels
>> (that I hard-coded above) that *appear* to be westward, and 0 to 360.
>> The entire map is still doing its math thing on a -180 to 180 east
>> grid.
>>
>> This becomes apparent when you draw a box (in my case, the viewing
>> area of the satellite instrument. If, for example the instrument is
>> viewing the box from lat,lon coordinates [-30,45] (bottom left corner)
>> to [0,0] to right corner:
>>
>> loadct, 39 & !p.color = 254 ; red box
>> plots, 45, -30, /data
>> plots, 45, 0, /data, /cont
>> plots, 0, 0, /data, /cont
>> plots, 0, -30, /data, /cont
>> plots, 45, -30, /data, /cont
>>
>> If you do this after issuing the first set of mapping commands above,
>> it appears as expected. But if you do it after the second map_set
>> command, it appears in the wrong place (the same place as before).
>>
>> ------------------------------------------------------------ ----------
>> Finally, if the above make sense, I will give you an exact description
>> of my problem. The above problem with drawing the box can be solved
>> easily, because a very simple function will translate between the four
>> possible grids (0-360 vs 180-180 vs 'east' vs 'west'). It would be
>> nice to specify this via a keyword *once* when I call map_set, and not
>> worry about it, but I can do the translations. But here is the real
>> problem:
>>
>> If you hardwire the longitude labels, you have to hardwire their
>> location (i.e. every degree or every 45 degrees, as above). My
>> software is used to view the surface of mars, and allows the user to
>> view it on a global scale, or down to a 1x1 degree grid.
>>
>> map_set, if allowed to 'do its own thing', will always generate nice
>> grids for you. If you are looking at the globe, it will use 45 degree
>> grids as shown above. If you are looking at a 1x1 degree grid, it will
>> use .1 degrees.
>>
>> But if I hardcode in the longitude names so they are westward, I also
>> have to code their locations. If I do every degree, you cannot see the
>> globe, all you see is the grid covering it. If I do every 45 degrees,
>> then you see no grids when you view a small area...
>>
>> Hence, my desire do use map_grid in a 'dumb' way (let it do the math
>> to determine the grid spacing), and have it output westward 0-360
>> grids.
>>
>> Finally, polar /ortho projections make this even more complicated than
>> equator-based /cylindrical projections.
>>
>> Sorry for the length of this post...
>>
>> -k.
>>
>> --
>> Kenneth Mankoff
>> LASP://303.492.3264
>> http://lasp.colorado.edu/~mankoff/
>> http://lasp.colorado.edu/snoe/
>> http://lasp.colorado.edu/mars/
>> http://lasp.colorado.edu/marsrobot/
>>
>>
>>
>>
>
>
>
--
Kenneth Mankoff
LASP://303.492.3264
http://lasp.colorado.edu/~mankoff/
http://lasp.colorado.edu/snoe/
http://lasp.colorado.edu/mars/
http://lasp.colorado.edu/marsrobot/
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