README.md

In adeckmyn/mapMaker: Create map databases from Natural Earth data

mapMaker

Create maps for the R package 'maps'

This is a package that helps to create maps that can be used with the R package 'maps'. Typical use could be to create a map with internal (state, provincial) borders for a particular country. Most likely input at this stage is data from the Natural Earth project.

Introduction

The 'maps' package comes with a number of maps installed, and the 'mapdata' packages adds several more. But sometimes people need e.g. a detailed map for a particular country, or an alternative version of a map. This package can help to make such data available for use with the 'maps' package. In this early stage, the main purpose is to read data from shapefiles available from the Natural Earth project (http://www.naturalearthdata.com).

This package provides:

• a simple routine that reads shape files (as provided e.g. by Natural Earth) and exports a list of polygons that can be directly used in 'map()'
• a set of R routines that transforms and exports this set of polygons into the file based format. This can even include splitting into polylines.
• export to ascii files (like in the sources of 'maps' and 'mapdata') or directly in binary format (as in the installed packages).

Formats

The data sets in 'maps' and 'mapdata' are distributed as ascii files, but installed in a particular binary format that allows for efficient reading of polygons and polylines.

However, the 'map()' function can also use a simpler map format which is not necessarily file based. A map database may also be a simple R list with three components: - x, y: two vectors containing longitude and lattitude of a set of polygons (with NA separating the polygons) - names: the names of the polygons

The output of map(...,fill=TRUE) is in fact such a list and can be used again:

myworld <- map('world',fill=TRUE)
map(myworld)

So the simplest way to use new data sets in 'maps' is as such a simple list of polygons. This is fine for many applications, but some of the possibilities of the 'maps' package are only available for the file based maps.

HOW TO

a typical mapMaker session may look like this:

# read provincial borders
be <- read.admin1(database="ne_10m_admin_1_states_provinces", countries="BE")
# you can use map() to see the result
map(be) 

To create a polyline-based set of files:

Either in a 1 line call:

be <- map.gon2line(be)

Or step by step:

1. Turn into an internal format, but still just polygons. This also does some /cleaning/ of the data: points that are equal up to the precision (default 1.E-8) are made numerically identical. Also, duplicate points that may arise are removed. On a large data set (e.g. 1:10 world database) this can take some time. be1 <- map.make(be)
2. Split the polygons into line segments consisting of 2 points. be2 <- map.split(be1)
3. Remove all duplicate segments (For large data sets this may take some time! About half a minute for the 1:50 map. For the 1:10 world map: have a coffee break). be3 <- map.dups(be2)
4. Calculate the 'valence' of every point (number of segments it belongs to). This may take some time (for the complete 1:50 world map from Natural Earth: a bit over 1 minute on my PC. For the 1:10 map it will be about 1 lunch break.). val <- map.valence(be3)
5. Shift the lines forming a polygon, until the polygon starts at a vertex (except islands, of course). be4 <- map.shift.gon(be3,val)
6. Merge all segments to polylines that begin and end at a vertex. be5 <- map.merge.segments(be4,val)
7. Fix which polygons are at the left and right of every polyline. be6 <- map.LR(be5)

To use this data set, we must export it to file(s) and make it available to 'map()'. This involves a special combination of an R variable giving the name of the environment variable that holds the path to the map data.

1. Export as a binary file. map.export.bin(be5,file="/tmp/belgium")
2. Make the map available to maps. This requires setting an environment variable and a local variable. Don't forget the trailing '/'. Sys.setenv("R_MY_MAP_PATH"="/tmp/") belgiumMapEnv <- "R_MY_MAP_PATH"
3. Finished! map(database="belgium")

CAVEAT

Turning the polygons into lines requires that the borders of polygons match exactly. That is often not the case. So the default is to first set all co-ordinates that are equal up to 8 decimals to be numerically equal. We do this as a first step, so in later steps we can assume common borders of polygons to be numerically identical. The binary format of 'maps' uses 32 bit floats, not double, so the default of 8 decimals seems reasonable.

TO DO

• improve documentation (roxygen2)
• use the data sets provide by rnaturalearth, rather than downloading yourself

adeckmyn/mapMaker documentation built on May 10, 2019, 5:52 a.m.