cshapes (version 0.6)

distlist: Compute distance lists on the CShapes dataset

Description

This function computes a distance list for the given date. It selects all the active CShapes polygons, determines their distances and outputs a distance list. A distance list is a list of dyads of countries and the distances between them. This list is returned as a data frame with three columns:

1. ccode1 -- country 1's code in the coding system specified by the useGW parameter

2. ccode2 -- country 2's code in the coding system specified by the useGW parameter

3. capdist, centdist, mindist -- distance between country 1 and country 2 in km, where distance can be either capital distance, centroid distance or minimum distance, as specified by the type parameter. The latter computation is very expensive if polygons have many nodes. For that reason, the function simplifies the country polygons according to the Douglas-Peucker algorithm (http://en.wikipedia.org/wiki/Ramer-Douglas-Peucker_algorithm), which eliminates points from the polygons and speeds up computation. The tolerance parameter specifies the tolerance for the simplification; a value of 0 disables it.

Note that the function returns directed dyads. For example, if there is a country with code 1 and a country with code 2, the resulting data frame contains the dyads (1,2), (2,1), (1,1) and (2,2). It is easy to extract the directed dyads from this data frame (see example below).

Usage

distlist(date, type="mindist", tolerance=0.1, useGW=T)

Arguments

date

The date for which the distance list should be computed. This argument must be of type Date and must be in the range 1/1/1946 - end of the dataset.

type

Specifies the type of distance list: capdist for capital distances, centdist for centroid distances, and mindist for minimum distances.

useGW

Boolean argument specifying the system membership coding. TRUE (Default): Gleditsch and Ward (1999). FALSE: Correlates of War.

tolerance

Tolerance for polygon simplification according the the Douglas-Peucker algorithm. Only used for mindist computation (type="mindist").

Value

A distance list with three columns, the first two of which contain the identifiers for the two countries in the dyad, and the third one containing the distance between the two countries.

Examples

# NOT RUN {
# Compute a list of minimum distances 
# for the international system on 1/1/1946
# using the Correlates of War list and the default accuracy
# }
# NOT RUN {
dl <- distlist(as.Date("1946-1-1"), type="capdist", tolerance=0.5, useGW=FALSE)
# }
# NOT RUN {
# we eliminate duplicate dyads
# }
# NOT RUN {
dl <- subset(dl, ccode1 < ccode2)
# }