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Re: [ferret_users] Fwd: reading ascii in polar stereographic projection

Well, the danger of this is that the SCAT2GRID functions are going to interpolate across the missing locations. In addition to creating spurious data where none should exist, this could end up polluting some real locations. For example if there is a narrow island (omitted from your list), then SCAT2GRID* will take values from one side of the island into account when computing a gridded value on the other side.

It sounds like you should think about fixing the column data before bringing it into Ferret.

Billy K

On May 8, 2009, at 9:39 AM, Ansley Manke wrote:

Hi Derek,
There are still some things you could do in Ferret even if you can't recover the complete grid from wherever this data came from. You could read in the longitudes, latitudes, and values into three 1- dimensional variables and use one of the SCAT2GRID* functions to put it onto a rectilinear grid.


Derek Tsui wrote:

Yes. The data I am working with has missing points. I believe they are missing data but instead of flagging them, the points were taken out of the file. So it is not a well-defined grid.

On Thu, May 7, 2009 at 5:55 PM, Ansley Manke <Ansley.B.Manke@xxxxxxxx> wrote:
Hi Derek,
I'm not sure how that could happen. For a well-defined polar stereographic grid, there has to be a complete set of longitudes and latitudes. There might be missing values for the data field, and those should have a missing-data flag, some particular value that marks that as bad data. Maybe what you're starting with has just left out those points?

Derek Tsui wrote:

Thanks Ansley. I was wondering what would I do then if nx and ny isn't fixed in the data? When there is missing data.

For example, xindex = 1, yindex = 1:140 , but xindex = 2, yindex = 1:135


On Tue, May 5, 2009 at 6:17 PM, Ansley Manke <Ansley.B.Manke@xxxxxxxx > wrote:
Hi Derek,
Your polar projection has coordinate data which is 2-dimensional, that is the longitudes and latitudes are defined on i,j index coordinate grid. At each i,j index pair there is a longitude, a latitude, and the value. Once you've accessed this data in Ferret, you'll use the curvilinear form of graphics calls to visualize the data.

What I would do is just write these three things to your ascii file, one longitude, latitude, value per record, with i varying fastest. Then define a grid which has x and y axes that are just the index values of the curvilinear coordinate variables: yes? let nx = 140 ! or whatever the number of values is for your grid
yes? let ny = 80

yes? define axis/x=1:`nx`:1 xindex
yes? define axis/y=1:`ny`:1 yindex

yes? define grid/x=xindex/y=yindex grid_ij

yes? file/var="xlon,ylat,value"/grid=grid_ij  myfile.dat

yes? FILL value, xlon, ylat

Derek Tsui wrote:

Sorry I'm not sure if there were a response since I sent out the question before subscribing.

---------- Forwarded message ----------
From: Derek Tsui <i.am.mr.sleepyhead@xxxxxxxxx>
Date: Wed, Apr 22, 2009 at 5:07 PM
Subject: reading ascii in polar stereographic projection
To: ferret_users@xxxxxxxx

I have a dataset in polar stereographic projection, ascii format, that I am trying to load into ferret and plot. It is 3 columns of data. longitutde, latitude, value. I can format it in any way.

I've tried the general script in the "Reading an ascii file: longitute latitute variable" thread but it doesn't seem to work in my case.

Any help would be much appreciated.

Thank you,

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