For the raster, my supervisor overlayed points in ArcGIS and I in R and we
got different attributes.  I think I will probably be studying that
difference in the long term and maybe we can work together?  Especially
since there are some people on this listserv from UC Davis!!

On Sun, Aug 30, 2009 at 9:37 PM, Robert J. Hijmans <r.hijmans@gmail.com>wrote:

> Hi Mehdi,
>
> > From what I understand, one
> > of the limitations is that there are tolerance limits within the program
> > which are used to assign points from one pixel to another.
>
> Where did you get that understanding? I know ArcGIS has various tolerance
> settings but I doubt it that these apply to this problem.
>
> > why would one need a tolerance when determining if a point is within a
> square or outside of it?
>
> No idea, and I do not think this need exists. Although there is a
> limitation to the floating point number precision that you can have on
> computers, in most cases this is about 16 digits; i.e. you are not likely to
> notice difference because of that very often.
>
> Apart from that, other sources of variation I can think of are (1) points
> on the edge of two or four raster cells (an arbitrary choice from these
> cells) (2) whether you pick the value in the cell or use some type of
> interpolation from the nearest cells to the point (in which case issue #1 no
> longer exists).
>
> Another reason of variation could possibly be caused by automatic
> adjustment if different datums are specified (perhaps ArcGIS does that).
>
> If you actually found variation between programs that you would like to
> understand, then I would be interested in looking at that, and others might
> be interested too, but we would need an example raster with problem points
> and the code that you have been using.
>
> Robert
>
>
> On Sun, Aug 30, 2009 at 6:29 PM, Mehdi Khan <mwkhan@ucdavis.edu> wrote:
>
>> Hello everyone, I am trying to investigate what the shortcomings may be of
>> using vectors/ rasters in determining attribute data from one program to
>> another.  My project has entailed overlaying points on different maps
>> (vector as well as raster based) and then determining attribute data from
>> each one and comparing them against one another.
>>
>> What would the limitations be in this process?  From what I understand,
>> one
>> of the limitations is that there are tolerance limits within the program
>> which are used to assign points from one pixel to another.  I don't fully
>> understand this concept-- why would one need a tolerance when determining
>> if
>> a point is within a square or outside of it?
>>
>> thank you all for your help!
>>
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>>
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>
>

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