[R-sig-Geo] Polygon width

[Paulo Flores Ribeiro]

Thank you, Barry! In the meanwhile, I've been exploring other paths and 
I think I found another solution, using loop. For a 
SpatialPolygonsDataFrame named "map4" the code was:

dmax <- data.frame()
for (i in 1:nrow(map4)) {
   poly = map4 at polygons[[i]]@Polygons[[1]]@coords
   dmax <- rbind.data.frame(dmax, max(dist(cbind(poly[,1],poly[,2]))))
}
print(dmax)

Anyway, I'll certainly try out your alternative solution, based on the 
(s)apply function, to see which one is most efficient.

Thank you so much again,

PauloFR


Às 18:16 de 25-04-2018, Barry Rowlingson escreveu:
> Loop over the row indexes of an sf-class object `pctest`, applying a 
> function given the row index which returns the distance:
>
>  > ppa = 
> sapply(1:nrow(pctest),function(i){max(st_distance(st_cast(st_geometry(pctest[i,]),"POINT")))})
>  > ppa
>   [1] 172.55598 360.77081 107.53889 137.17785  51.66645 132.82052 
> 113.00875
>   [8] 161.02432 141.13909  88.67002
>
> If you want to make that a new column, do `pctest$maxdist = ppa`
>
> And test on a simple example - make sure a unit square returns 
> approximately sqrt(2)!
>
> Barry
>
>
>
>
> On Wed, Apr 25, 2018 at 5:24 PM, Paulo Flores Ribeiro 
> <paulo.flores.mail at gmail.com <mailto:paulo.flores.mail at gmail.com>> wrote:
>
>     Thank you, Barry. I am using planar coordinates (meters) and
>     Euclidean distances. The shape or size of the polygons is not
>     important (for illustration purposes, imagine that polygons are
>     farm boundaries and that each farm is a single polygon). I know
>     how to extract the maximum distance in a single polygon by
>     calculating the distance matrix and selecting the maximum value.
>     My "difficulty" is exactly in the coding of the loop process (or
>     in the construction of the "function" to be used in the apply
>     approach), so that I can apply it "automatically" to 25,000
>     polygons. I am using the rgadl package, but I can switch to sf.
>     Thanks for any help.
>     Cheers,
>     PauloFR
>
>     Às 12:27 de 25-04-2018, Barry Rowlingson escreveu:
>>     Do you want great-circle distance or is your space small enough
>>     that you can use planar coordinates?
>>
>>     Are your polygons all single rings or are there islands and/or
>>     holes? Does that matter?
>>
>>     The straightforward way would be to coerce the polygons to
>>     points, compute the distance matrix, then take the maximum.
>>     Depending on if you are reading your shapefile into sp or sf
>>     classes, the functions would be a bit different. You should try
>>     and implement the straightforward way, test it for correctness,
>>     and then worry about the "best way" if the straightforward way
>>     isn't what you need. Its often the case that "best" ways need
>>     fancy data structures or complex algorithms with more opportunity
>>     for bugs. Start simple, work up.
>>
>>     For example, using sf, here's the max distance between any points
>>     in the first feature of `pcs`
>>
>>     > max(st_distance(st_cast(st_geometry(pcs[1,]),"POINT")))
>>     172.556 m
>>
>>     loop from 1 to N or otherwise apply over the features, and you're
>>     done.
>>
>>     Barry
>>
>>
>>
>>     On Wed, Apr 25, 2018 at 11:26 AM, Paulo Flores Ribeiro
>>     <paulo.flores.mail at gmail.com
>>     <mailto:paulo.flores.mail at gmail.com>> wrote:
>>
>>         Hello,
>>
>>         I have a shapefile with ca. 25000 polygons. Each polygon has
>>         an average of 40 vertices (nodes). I would like to extract,
>>         for each polygon, the distance separating the two most
>>         distant vertices (aka "polygon diagonal" or "maximum polygon
>>         width"). It is not important whether the polygon is convex or
>>         concave, so the lines connecting the vertices can be inside
>>         or outside the polygon. The desired result would be a
>>         two-column array, with a number of rows equal to the number
>>         of polygons, and where the first column is the id of the
>>         polygons, and the second the "maximum width" of the
>>         corresponding polygon.
>>
>>         What would be the best way to do this, considering that the
>>         calculation will probably require frequent updates (e.g. due
>>         to changes in the shape of the polygons)?
>>
>>         Thanks in advance,
>>
>>         PauloFR
>>
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