GIS data structures are not well suited for generalization, and visualizations and models in 3D require pretty forceful and ad hoc approaches.
Here I describe a simple example, showing several ways of visualizing a simple polygon data set. I use the programming environment R for the data manipulation and the creation of this document via several extensions (packages) to base R.
Polygon “layer”
The R package maptools contains an in-built data set called wrld_simpl, which is a basic (and out of date) set of polygons describing the land masses of the world by country. This code loads the data set and plots it with a basic grey-scale scheme for individual countries.
class : SpatialPolygonsDataFrame
features : 246
extent : -180, 180, -90, 83.57027 (xmin, xmax, ymin, ymax)
variables : 11
# A tibble: 246 × 11
FIPS ISO2 ISO3 UN NAME AREA POP2005 REGION SUBREGION LON LAT
<fct> <fct> <fct> <int> <fct> <int> <dbl> <int> <int> <dbl> <dbl>
1 AC AG ATG 28 Antigu… 44 8.30e4 19 29 -61.8 17.1
2 AG DZ DZA 12 Algeria 238174 3.29e7 2 15 2.63 28.2
3 AJ AZ AZE 31 Azerba… 8260 8.35e6 142 145 47.4 40.4
4 AL AL ALB 8 Albania 2740 3.15e6 150 39 20.1 41.1
5 AM AM ARM 51 Armenia 2820 3.02e6 142 145 44.6 40.5
6 AO AO AGO 24 Angola 124670 1.61e7 2 17 17.5 -12.3
7 AQ AS ASM 16 Americ… 20 6.41e4 9 61 -171. -14.3
8 AR AR ARG 32 Argent… 273669 3.87e7 19 5 -65.2 -35.4
9 AS AU AUS 36 Austra… 768230 2.03e7 9 53 136. -25.0
10 BA BH BHR 48 Bahrain 71 7.25e5 142 145 50.6 26.0
# … with 236 more rows
plot(wrld_simpl, col =grey(sample(seq(0, 1, length =nrow(wrld_simpl)))))
We also include a print statement to get a description of the data set, this is a SpatialPolygonsDataFrame which is basically a table of attributes with one row for each country, linked to a recursive data structure holding sets of arrays of coordinates for each individual piece of these complex polygons.
These structures are quite complicated, involving nested lists of matrices with X-Y coordinates. I can use class coercion from polygons, to lines, then to points as the most straightforward way of obtaining every XY coordinate by dropping the recursive hierarchy structure to get at every single vertex in one matrix.
(There are other methods to obtain all coordinates while retaining information about the country objects and their component “pieces”, but I’m ignoring that for now.)
We need to put these “X/Y” coordinates in 3D so I simply add another column filled with zeroes.
(Note for non-R users: in R expressions that don’t include assignment to an object with <- are generally just a side-effect, here the side effect of the head(allcoords) here is to print the top few rows of allcoords, just for illustration, there’s no other consequence of this code).
OpenGL in R
In R we have access to 3D visualizations in OpenGL via the rgl package, but the model for data representation is very different so I first plot the vertices of the wrld_simpl layer as points only.
library(rgl)plot3d(allcoords, xlab ="", ylab ="") ## smart enough to treat 3-columns as X,Y,Zrglwidget()
Plotting in the plane is one thing, but more striking is to convert the vertices from planar longitude-latitude to Cartesizan XYZ. Define an R function to take “longitude-latitude-height” and return spherical coordinates (we can leave WGS84 for another day).
llh2xyz <-function (lonlatheight, rad =6378137, exag =1) { cosLat =cos(lonlatheight[, 2] * pi/180) sinLat =sin(lonlatheight[, 2] * pi/180) cosLon =cos(lonlatheight[, 1] * pi/180) sinLon =sin(lonlatheight[, 1] * pi/180) rad <- (exag * lonlatheight[, 3] + rad) x = rad * cosLat * cosLon y = rad * cosLat * sinLon z = rad * sinLatcbind(x, y, z)}## deploy our custom function on the longitude-latitude valuesxyzcoords <-llh2xyz(allcoords)
Now we can visualize these XYZ coordinates in a more natural setting, and even add a blue sphere for visual effect.
