[UW-GIS-L] projections and POINTGRID

Thu Feb 27 08:33:27 PST 2003

hi Harvey - 
I received the data as lat/long/ice in decimal degrees. This file is unprojected. However, the programmers report the data were sampled and recorded in a Polar Stereographic projection over the entire Arctic, and converted to geographic for users to download. When I convert this file to a coverage in Arc and reproject it into Polar using the stated projection parameters from the ice agency, the point coverage wraps over the Arctic correctly and the points are spaced equally 25 km apart (give or take a few meters). I don't think the parameter values are incorrect. Note though, that when you look at the pattern of this point coverage, the points fall in an equally spaced DIAGONAL plane across my study area.

I want to create a Polar grid from this projected point coverage. I thought the 'pointgrid' function would work because I just want each point to represent the CENTER of each pixel, average sea ice concentration in a 25 sq km cell. I don't want to change the resolution. When I run 'pointgrid' on the projected file, I get a grid - but it does not have the points in the center of each pixel. The grid appears to be created in a different plane than my points (a horizontal-vertical plane), which consequently "bins" more than one point into some cells and "no points" into other cells. It appears to be shifted by 45 degrees or something odd. 

So I want to project pointcov C (geographic) to create pointcov B (polar) and then create grid A (polar). 

Thanks for taking the time to discuss this. Kristin

-----------------------------------
Kristin Laidre
Washington Cooperative Fish and Wildlife Research Unit
School of Aquatic and Fishery Sciences
Box 355020
University of Washington
Seattle, Washington 98195 USA

Phone (NMML) 206.526.6866
Email: klaidre at u.washington.edu

On Wed, 26 Feb 2003, Harvey Greenberg wrote:

> I hear your question differently.
> So there was:
> A: a polar grid
> B: a point version of A
> C: a reprojected version of B, which you received.
> 
> and you want to reproject C back to match B so you can recreate A?
> But you are not recreating B?
> If this is your problem, there is something wrong with the parameters
> you were given for the polar projection or the way you communicated
> the parameter values to ARC.  Show us your work.
> 
> harvey
> 
> klaidre at u.washington.edu wrote:
> > Hi everyone - 
> > 
> > Is anyone familiar with issues pertaining to projected point data and raster conversion, especially in a Polar projection? I'm working in Arc/GRID writing an aml to convert ASCII point data to a raster, where each point should be the CENTER of a pixel representing a 25x25 sq km mean sea ice concentration value in the circumpolar Arctic. The SSMI sea ice data were collected daily by the satellite in Polar Stereographic, yet made available to users in Geographic-decimal degrees.
> > 
> > My aml takes the ascii point data and ice value (x, y, z) - creates a point coverage, projects the point coverage into Polar, and then uses the POINTGRID function to create a raster in 25km res. The problem is - I end up with a grid containing some cells that look ok, but also multiple empty cells with "no data" containing no points, as well as many single cells with more than two points! It seems that the POINTGRID f(x) creates the grid in an x-y plane that is not the same as the x-y plane of the polar projected points (kind of on a 45 degree diagonal). How can I create this raster so that the mean values represent the center of each cell?
> > 
> > Thanks! Kristin
> > 
> > -----------------------------------
> > Kristin Laidre
> > Washington Cooperative Fish and Wildlife Research Unit
> > School of Aquatic and Fishery Sciences
> > Box 355020
> > University of Washington
> > Seattle, Washington 98195 USA
> > 
> > Phone (NMML) 206.526.6866
> > Email: klaidre at u.washington.edu
> > 
> > 
> > 
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> -- 
> hgreen at u.washington.edu  162 Johnson Hall  206-685-7981	
> http://duff.ess.washington.edu/
> 
> 	Harvey Greenberg
> 	University of Washington
> 	Department of Earth and Space Sciences
> 	63 Johnson Hall (Box 351310)
> 	Seattle WA 98195-1310
> 
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