[R-sig-Geo] Convert SpatialPointsDataFrame to SpatialPolygons or SpatialPolygonsDataFrame
Miluji Sb
milujisb at gmail.com
Mon Dec 26 13:32:56 CET 2016
Dear Michael,
Thank you again for your reply. Apologies for not answering your question
earlier. My goal is to extract data a GeoTIFF file using the nightlights
package. The requirement of the package is that shapefile must be in a
SpatialPolygons or SpatialPolygonsDataFrame. Thanks!
Sincerely,
Milu
On Mon, Dec 26, 2016 at 1:12 PM, Michael Sumner <mdsumner at gmail.com> wrote:
>
>
> On Mon, 26 Dec 2016 at 22:39 Miluji Sb <milujisb at gmail.com> wrote:
>
>> Dear Michael,
>>
>> Merry Christmas and seasons's greetings! Thank you for your reply, I am
>> providing more information and my attempts to convert
>> to SpatialPolygonsDataFrame below.
>>
>> The original SpatialPointsDataFrame has the following attributes:
>>
>> class : SpatialPointsDataFrame
>> features : 21441
>> extent : -179.5, 179.5, -89.5, 83.5 (xmin, xmax, ymin, ymax)
>> coord. ref. : +proj=longlat +ellps=WGS84
>> variables : 3
>> names : ID_cell, lon, lat
>> min values : 2304, -179.5, -89.5
>> max values : 64800, 179.5, 83.5
>>
>> The data looks like this:
>>
>> dput(head(shp at data,20))
>> structure(list(ID_cell = c(2304L, 2305L, 2306L, 2307L, 2308L,
>> 2309L, 2310L, 2311L, 2618L, 2619L, 2620L, 2621L, 2622L, 2623L,
>> 2624L, 2625L, 2626L, 2627L, 2628L, 2629L), lon = c(-36.5, -35.5,
>> -34.5, -33.5, -32.5, -31.5, -30.5, -29.5, -82.5, -81.5, -80.5,
>> -79.5, -78.5, -77.5, -76.5, -75.5, -74.5, -73.5, -72.5, -71.5
>> ), lat = c(83.5, 83.5, 83.5, 83.5, 83.5, 83.5, 83.5, 83.5, 82.5,
>> 82.5, 82.5, 82.5, 82.5, 82.5, 82.5, 82.5, 82.5, 82.5, 82.5, 82.5
>> )), .Names = c("ID_cell", "lon", "lat"), data_types = c("N",
>> "F", "F", "C", "C", "C", "C"), row.names = c("0", "1", "2", "3",
>> "4", "5", "6", "7", "8", "9", "10", "11", "12", "13", "14", "15",
>> "16", "17", "18", "19"), class = "data.frame")
>>
>> I tried the following:
>>
>> x = as(SpatialPixelsDataFrame(shp, shp at data, tolerance=.00086),
>> "SpatialPolygonsDataFrame"), it seemed to have worked but got the following
>> warning:
>>
>> Warning message:
>> In points2grid(points, tolerance, round) :
>> grid has empty column/rows in dimension 2
>>
>> Now the converted SpatialPolygonsDataFrame has the following attributes:
>>
>> class : SpatialPolygonsDataFrame
>> features : 21441
>> extent : -180, 180, -90, 84 (xmin, xmax, ymin, ymax)
>> coord. ref. : +proj=longlat +ellps=WGS84
>> variables : 3
>> names : ID_cell, lon, lat
>> min values : 2304, -179.5, -89.5
>> max values : 64800, 179.5, 83.5
>>
>> structure(list(ID_cell = c(2304L, 2305L, 2306L, 2307L, 2308L,
>> 2309L, 2310L, 2311L, 2618L, 2619L, 2620L, 2621L, 2622L, 2623L,
>> 2624L, 2625L, 2626L, 2627L, 2628L, 2629L), lon = c(-36.5, -35.5,
>> -34.5, -33.5, -32.5, -31.5, -30.5, -29.5, -82.5, -81.5, -80.5,
>> -79.5, -78.5, -77.5, -76.5, -75.5, -74.5, -73.5, -72.5, -71.5
>> ), lat = c(83.5, 83.5, 83.5, 83.5, 83.5, 83.5, 83.5, 83.5, 82.5,
>> 82.5, 82.5, 82.5, 82.5, 82.5, 82.5, 82.5, 82.5, 82.5, 82.5, 82.5
>> )), .Names = c("ID_cell", "lon", "lat"), data_types = c("N",
>> "F", "F", "C", "C", "C", "C"), row.names = c("g144", "g145",
>> "g146", "g147", "g148", "g149", "g150", "g151", "g458", "g459",
>> "g460", "g461", "g462", "g463", "g464", "g465", "g466", "g467",
>> "g468", "g469"), class = "data.frame")
>>
>> Is this correct or even on the right track? I hope I have provided enough
>> information. Thank you so much.
>>
>>
> That is certainly one correct conversion of a set of points, interpreted
> as a regular grid of cells about the points-as-centres, then converted to
> polygons (quite different from the wrong guess I made at first). It's
> extremely inefficient because of all the redundant coordinates stored for
> every polygon, but that may not be a concern.
>
> Without knowing your requirements it's not possible to know if this is a
> useful way to go. You can avoid the redundancy by keeping the thing as a
> grid, and I would explore the raster package for an alternative to the
> SpatialPixelsDataFrame used here, it doesn't allow mixed columns of data
> types on a data frame but does have a lot of high-level tools for regular,
> numeric grids.
>
> We still don't know why you wanted polygons, or whether the tolerance of
> "regular" used will affect the results you are after but other than that
> and the inefficiencies inherent in the storage I don't see any real problem
> with what you're doing.
>
> If you can detail why you want to do this, and what comes next, there's
> probably better advice to be given.
>
> Cheers, Mike.
>
> SIncerely,
>>
>> Milu
>>
>> On Mon, Dec 26, 2016 at 7:24 AM, Michael Sumner <mdsumner at gmail.com>
>> wrote:
>>
>> It is but you will need at least a grouping ID for each polygon ring
>> within each object, and an order within each ring. Is ID_cell the object ID?
>>
>> If there is only a single ring polygon within each object and the order
>> is native it's simplest, but the constructors require very particular
>> arrangements of your data, there is no abstractions for this. See
>> raster::spPolygons and spbabel::sp for alternative constructors, but other
>> wise see Polygon, Polygons, SpatialPolygons and SpatialPolygonsDataFrame in
>> sp.
>>
>> There's not enough information in your post to be sure of what you have,
>> but I'm happy to help if you provide more detail about how your data is
>> organized. You could post sample data, or describe them in more detail. Do
>> you have actual polygons here or are you attempting to estimate them from
>> point data?
>>
>> To add more alternatives the new sf package simplifies the constructors a
>> lot, for a new family of classes, but still you need to nest matrices of
>> coordinates manually to use them rather than specify their organization
>> abstractly. I'm working on tools to do that, but there is only spbabel so
>> far (it's really just a two-level grouping of coordinates for spatial
>> objects and their parts, but various legacies make it ever more
>> complicated).
>>
>> Cheers, Mike
>>
>> On Sun, Dec 25, 2016, 02:28 Miluji Sb <milujisb at gmail.com> wrote:
>>
>> I have a SpatialPointsDataFrame with the following attributes. Is it
>> possible to convert this to a SpatialPolygons or SpatialPolygonsDataFrame?
>> Thank you!
>>
>> Sincerely,
>>
>> Milu
>>
>> class : SpatialPointsDataFrame
>> features : 21441
>> extent : -179.5, 179.5, -89.5, 83.5 (xmin, xmax, ymin, ymax)
>> coord. ref. : +proj=longlat +a=6367470 +b=6367470 +no_defs
>> variables : 7
>> names : ID_cell, lon, lat, Field4, Field5, Field6, Field7
>> min values : 2304, 0.5, 0.5, NA, NA, NA, NA
>> max values : 64800, -179.5, -89.5, NA, NA, NA, NA
>>
>> [[alternative HTML version deleted]]
>>
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>>
>> --
>> Dr. Michael Sumner
>> Software and Database Engineer
>> Australian Antarctic Division
>> 203 Channel Highway
>> Kingston Tasmania 7050 Australia
>>
>>
>> --
> Dr. Michael Sumner
> Software and Database Engineer
> Australian Antarctic Division
> 203 Channel Highway
> Kingston Tasmania 7050 Australia
>
>
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