[R-sig-Geo] Accuracy and limitations of Raster and Vector maps

[Edzer Pebesma]

Mehdi Khan wrote:
> For that specific problem, this is what I ended up doing:
>
> wald <- SpatialPixelsDataFrame(coordinates(wald),
> as.data.frame(wald)[,3,drop = FALSE], tolerance = 0.125)
>
> So what does this mean exactly?  that since it wasn't an exact grid, a
> tolerance was specified to "straighten" curves into a grid structure? 
Yes.

If you want to compare the coordinates, safe the result to wald1 (so
that wald is still there), and compare coordinates(wald1) to
coordinates(wald).

If you want to see what went on, type points2grid and you'll see what
the function did.
--
Edzer
>  I
> appreciate your help, my experience only goes back 8 weeks or so with this
> program.
>
>
> Mehdi Khan
>
>
> On Mon, Aug 31, 2009 at 12:37 AM, Edzer Pebesma <
> edzer.pebesma at uni-muenster.de> wrote:
>
>   
>> I am not sure if I can follow your reasoning. The function
>> SpatialGridDataFrame tries to form a grid from a set of points. In the
>> earlier example output you sent, it seemed that the set of points you
>> had were not exactly on a grid. Look at the following two examples:
>>
>>     
>>> library(sp)
>>> pts1 = data.frame(x = rnorm(10),y = rnorm(10), z = rnorm(10))
>>> gridded(pts1) = ~x+y
>>>       
>> suggested tolerance minimum: 0.758031516672908
>> Error in points2grid(points, tolerance, round, fuzz.tol) :
>>  dimension 1 : coordinate intervals are not constant
>>     
>>> pts2 = data.frame(expand.grid(x=1:10,y=1:10),z=rnorm(100))
>>> gridded(pts2) = ~x+y
>>> points2grid(pts2)
>>>       
>>                   x  y
>> cellcentre.offset  1  1
>> cellsize           1  1
>> cells.dim         10 10
>>
>> It is clear that for pts1 it is complete nonsense to try to find a grid
>> structure, as it is not there. For the second one it is there by
>> construction -- expand.grid does this.
>>
>> For cases where data are almost on a grid, but not exactly (often
>> because of rounding of coordinates that are not round numbers but
>> written to ascii text), points2grid tries to find a useful grid
>> topology, but may fail. You can enlarge its tolerance to help it find
>> one, but no success guaranteed.
>>
>> If you want to compare overlays, you better make sure you have a real
>> and identical grid in both environments (R and ArcGIS), and do the
>> overlay in both. In R, this is done with the overlay method.
>> --
>> Edzer
>>
>> Mehdi Khan 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!
>>>
>>>       [[alternative HTML version deleted]]
>>>
>>> _______________________________________________
>>> R-sig-Geo mailing list
>>> R-sig-Geo at stat.math.ethz.ch
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>       
>> --
>> Edzer Pebesma
>> Institute for Geoinformatics (ifgi), University of Münster
>> Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
>> 8333081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de/
>> http://www.springer.com/978-0-387-78170-9 e.pebesma at wwu.de
>>
>>     
>
>   

-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de/
http://www.springer.com/978-0-387-78170-9 e.pebesma at wwu.de