R help - Apr 2011 - nndist R vs. ArcGIS

Can anyone tell me why I would get different average nearest neighbor values
for the same set of coordinates between ArcGIS 10 and R? Sometimes the
difference in distance is over 1.3 km.

Alexis



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Alexis wrote:
> Can anyone tell me why I would get different average nearest neighbor 
values
> for the same set of coordinates between ArcGIS 10 and R? Sometimes the
> difference in distance is over 1.3 km.
spatstat::nndist calculates Euclidean distances rather than distances 
along the earth's surface, which is probably what you're getting from
AG.
A very short example illustrating the problem would be helpful in 
determining if I'm at all right; on the other hand I'm glad you're
testing
your results. 

You might peek at the current thread "Geographic distance between lat-long 
points in R" for more information.

cur
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Curt Seeliger, Data Ranger
Raytheon Information Services - Contractor to ORD
seeliger.curt@epa.gov
541/754-4638

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On Mon, Apr 11, 2011 at 4:49 PM, smoluka <smoluka at geo.oregonstate.edu>
wrote:
> Can anyone tell me why I would get different average nearest neighbor
values
> for the same set of coordinates between ArcGIS 10 and R? Sometimes the
> difference in distance is over 1.3 km.
 Edge correction? In a spatial point pattern, points near the boundary
of your window are less likely to have a near neighbour because only
some of the surrounding space can possibly have points. I think
functions in spatstat will correct for this.

Make a simple test example and tell us what functions you are using.

And also try the r-sig-geo mailing list  for this sort of thing.

Barry

On 12/04/11 07:32, Barry Rowlingson wrote:
> On Mon, Apr 11, 2011 at 4:49 PM, smoluka<smoluka at
geo.oregonstate.edu>  wrote:
>> Can anyone tell me why I would get different average nearest neighbor
values
>> for the same set of coordinates between ArcGIS 10 and R? Sometimes the
>> difference in distance is over 1.3 km.
>   Edge correction? In a spatial point pattern, points near the boundary
> of your window are less likely to have a near neighbour because only
> some of the surrounding space can possibly have points. I think
> functions in spatstat will correct for this.
         No.  Not as far as I am aware or can discern.  The function
         nndist() does ***not*** invoke any edge correction.  It simply
         calculates the distances as they are, for the points that appear
         in the window, and takes the appropriate minima.

             cheers,

                 Rolf Turner
> Make a simple test example and tell us what functions you are using.
>
> And also try the r-sig-geo mailing list  for this sort of thing.
>
> Barry

The problem was on the ArcGIS 10 end.  The program corrupted my layers.  I
re-created the layer and now I get the same answer.  Thank  you all for your
insight.  

Alexis

PS, the scale is in terms of owl territories and therefore 1.3 km is really
huge.  

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