| Map image with a sparse array [message #18662] |
Fri, 21 January 2000 00:00  |
whdaffer
Messages: 10
Registered: January 2000
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Junior Member |
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Hi;
I have the following problem. I have an array of geographically
colocated data (0.5 by 0.5 degree grid) that is the result of averaging
all 253 swaths of one cycle of Topex data into this grid. Topex data has
a small (3 or maybe 10km) swath, so the majority of the grid location
(65%) are 'bad' in the sense that those grid elements contain no
averaged data.
I want to display this by mapping it using map_set/map_image. The old
method simply 'tv'd the image to the screen and then finessed applying
the continents/grid lines to the image. A bit of a boondogle, and not
very upgradeable.
The problem is that there seems to be no way to tell map_image (and
map_patch too) that certain data (the 'bad' data value) should be
excluded from whatever
averaging/bilinear-interpolation/nearest-neighbor-chosing method is used
and the 'mapped' image has places that are clearly corrupted by the
presence of the bad data. The problem is ameliorated by use nearest
neighbor rather than bilinear interpolation (i.e. bilinear=0) and I am
setting compress=0, so that the inverse transformation is done on each
pixel. Also, I've started out with a window set to the size of the input
data array and with map_set,position=[0.,0,1,1] so that the mapping
coordinate system occupies the entire window. These remedies I hit upon
thinking that they would minimize the damage, and they have done that,
but when I compare my results with the older, more 'pristine' but vastly
less portible, upgradeable, maintainable method, there are big
differences.
The 'missing' keyword just sets elements outside the range input via
the 'min' and 'max' keywords and those outside of the mapping
coordinates to the bad value, it doesn't allow one to exclude data from
the averaging/interpolation/chosing method.
Will I have to hack map_image? Or go back to the old way?
William
Sent via Deja.com http://www.deja.com/
Before you buy.
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| Re: Map image with a sparse array [message #18758 is a reply to message #18662] |
Thu, 27 January 2000 00:00  |
William Daffer
Messages: 34
Registered: February 1999
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Member |
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"Liam E. Gumley" <Liam.Gumley@ssec.wisc.edu> writes:
> whdaffer@my-deja.com wrote:
>
>> Can anyone see what I'm doing wrong?
>>
>> When I tv the mapped image and compare it to the unmapped image, it's
>> clear that the mapped image has included some 'bad' data in the
>> calculations of points within the grid.
>>
>> To be fair, this possibility is not explicitly excluded in the help
>> file. If I'm reading it correctly, all it says is that output grid
>> values which are within the valid mapping limits but which exceed input
>> maxima will be set to the 'bad data' value, not that the input 'bad
>> data' value will be excluded from caluclations.
>>
>> The effect is not large, but it isn't small either. The person I'm
>> doing the work for noticed it right off.
>>
>> By the way, I did try your method first, (bytscl, then map_image) and
>> got similar results, but thought that I should make the bytscl be the
>> last step, since it reduces the 'color' resolution from 360 values to
>> about 200. I'll go back and try your method again. Perhaps there's some
>> magic that I'm not seeing.
>
> William,
>
> I pulled that section of code from a procedure that projects a 720x360
> global grid with missing data onto various map projections. I cannot see any
> hint that the missing data corrupted the resulting image:
> ftp://origin.ssec.wisc.edu/pub/gumley/IDL/grid_project002.gi f
>
> Cheers,
> Liam.
>
Liam;
It looks like your array is considerably more filled-out than
mine.
Go to ftp://catspaw.jpl.nasa.gov/pub/sparse-array
The three files therein are:
h_mss_c249-v1.jpg : map_image then bytscl
h_mss_c249-v2.jpg : bytscl then map_image
tarray.gif : The data, array size unchanged,
bytscl'd and tv'd only
The colored lines in tarray.gif are the data, the grey background
are the grid elements with no data.
I keep meaning to have a close look at map_image, but other
exigencies have intruded. I'm not at all sure that this isn't a
problem of my own making.
William
--
Outside of a dog, a book is man's best friend.
Inside of a dog, it's too dark to read.
Groucho Marx.
Public Key: http://home.earthlink.net/~whdaffer/#PGP-public-key
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