Chusi Kuroiwa

Susumu Tanimura

Tsutomu Mizota

English

Research Papers in Economics

JSS Journal of Statistical Software
April 2007, Volume 19, Code Snippet 1. http://www.jstatsoft.org/
Auxiliary Cartographic Functions in R: North
Arrow, Scale Bar, and Label with a Leader Arrow
Susumu Tanimura
Nagasaki University
Chusi Kuroiwa
University of Tokyo
Tsutomu Mizota
Nagasaki University
Abstract
Auxiliary cartographic components such as a scale bar, a north arrow, and labels
frequently play an important role in faciliating the use of maps. Since the current R
environment does not fully support these tools, we developed a scale bar for a projected
map, a north arrow, and interactive text labeling with a leader arrow. The functionalities
of the developed codes and noteworthy points during the execution of these functions are
discussed.
Keywords: cartography, thematic map, R, scale bar, north arrow, leader arrow.
1. Introduction
Cartographic expressions of spatial statistical data are important for imparting spatial con-
cepts or ideas. Since using only shapes and colors of objects in a map is not always suﬃcient
to lucidly represent them, we use supplementary components such as a legend, a scale bar, a
north arrow, and text labels (Slocum, McMaster, Kessler, and Howard 2005).
Although these components play an important role for map users to understand the map,
cartographic functions in R (R Development Core Team 2007) do not include scale bars
for a projected map, north arrows, and interactive text labeling except the two functions
layout.north.arrow() and layout.scale.bar() in package sp.
In this study, we develop the abovemetioned three auxiliary functions by using the R language.
This paper explains the functionality of each tool and illustrates the result of the execution.
2. Labeling
It is troublesome to refer to a speciﬁc location on a map without using the label of the location,2 Auxiliary Cartographic Functions in R
particularly when the location is indicated in a document.
The function text() in R can be used to place text on the graphic device. For example, we
can use text() to automatically locate every name attribute of a polygon on the centroid
calculated by get.Pcent() from maptools (Lewin-Koh and Bivand 2007). However, in many
cases, the labels become congested and overprinted causing the text to be unsightly and
unreadable. In such a case, in order to cull the essential labels referred to in the description,
we manually reuse text() for each selected label. However, there arises another problem we
must know the label content prior to its placement. Since dealing with a linkage between a
location on a map and an attribute value is typically diﬃcult, only users who are certain of
both the location and attribute value can use text() for such labeling.
Another solution to avoid the overprint of labels is to move them to an outer place using a
leader line or an arrow. However, this has the same disadvantage for the abovementioned
labeling.
We develop an interactive labeling function map.arrow.label() using the R language, which
enables the placement of a label on a polygon map with a leader arrow without the prior
knowledge of its content. We internally use linkage between map polygons and attribute
values in the application of inout() in the splancs package (Rowlingson, Diggle, and Bivand
2006). The map.arrow.label() code is as follows:
map.arrow.label <- function(map=NULL,label,n.label=1,length=.1,cex=.8,
box.margin=.1,bgcol="white",tcol="black",
adj=c(.5,.5),iconv.from=NULL,iconv.to=NULL,...)
{
require(splancs)
if (class(map)!="Map")
stop("map is not Map object.")
if (attr(map$Shapes, "shp.type") != "poly")
stop("Type of map is not polygon.")
if (missing(label))
stop("label is missing.")
## count total polygons
poly.n <- length(map$Shapes)
j <- 0
while (j < n.label) {
## get location of target and label
cat("Click now the location of polygon and then label\n")
loc <- unlist(.Internal(locator(2, type ="n")))[c(1,3,2,4)]
## draw the leader arrow
arrows(loc[1],loc[2],loc[3],loc[4],length=length,code=1)
## find the content of the label
loc1 <- matrix(c(loc[1],loc[2]),ncol=2)
poly.inout <- vector(mode = "logical", length = poly.n)
for (i in 1:poly.n) {
poly.inout[i] <- inout(loc1,map$Shapes[[i]]$verts)
}
labels <- map$att.data[,which(names(map$att.data) == label)]
thelabel <- as.character(labels[poly.inout])
if(is.na(thelabel)) warning("The point may be out of range.")
## convert an encoding
if(length(iconv.from)!=0 & length(iconv.to)!=0) {
thelabel <- iconv(thelabel,iconv.from,iconv.to)Journal of Statistical Software – Code Snippets 3
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Figure 1: An example of labeling with a leader arrow using map.arrow.label(). The
Japanese text in the labels was converted from sjis to euc-jp
}
## draw a rectangle
thelabel.size <- c(strwidth(thelabel), strheight(thelabel))
rect(loc[3]-thelabel.size[1]/(2-box.margin),
loc[4]-thelabel.size[2]/(2-box.margin),
loc[3]+thelabel.size[1]/(2-box.margin),
loc[4]+thelabel.size[2]/(2-box.margin),
col=bgcol)
## place the text of label
text(loc[3],loc[4],thelabel,adj=adj,cex=cex,col=tcol)
j <- j+1
}
}
For execution, users must specify the map as a base map to be labeled; a base map of this
type should exclusively be a “Map object” in the maptools package for simpliﬁcation. Users
also must specify the name of variable for label text by label option. Note that it must be
one of attributes in the Map object and speciﬁed in quotes. Users can repeatedly place labels
without user intervention any number of times, which is speciﬁed by n.label option (the
default value is one).4 Auxiliary Cartographic Functions in R
The label properties can be adjusted by using options such as length for the length of the
edges of the arrow head (in inches), cex as a multiple proportion for the size of a label text,
box.margin for the margin of the text in a box, adj for the adjustment of the text position,
and bgcol and tcol for the background and text color in the label, respectively. A default
label shows black letters against a white background in a rectangular box.
The usage of map.arrow.label() is simple and as follows: click on a target polygon with
a mouse; then, click again on the location where the label is to be placed. The steps are
repeated if the user speciﬁes a value for n.label. An example output of map.arrow.label()
is shown in Figure 1.
The code may face problems when a map has multi-polygons because it assumes a one-to-one
matching of the data. A signiﬁcant improvement is required for handling such data, and we
intend focusing on it in the future.
Note that users who need to convert an encoding in the text of a label can use the options
iconv.from and iconv.to in map.arrow.label(). This function is essential for users using
a language that involves more than one encoding.
3. North arrow
Map users who attempt to ﬁnd north at the top of a map might be confused due to the absence
of an indication of direction such as a north arrow or a graticule on the map. Nevertheless,
maps are occasionally rotated due to the shape of the map and/or the mapped space.
In order to provide the direction on the map for R users, we developed the north arrow
function northarrow() as follows:
northarrow <- function(loc,size,bearing=0,cols,cex=1,...) {
# checking arguments
if(missing(loc)) stop("loc is missing")
if(missing(size)) stop("size is missing")
# default colors are white and black
if(missing(cols)) cols <- rep(c("white","black"),8)
# calculating coordinates of polygons
radii <- rep(size/c(1,4,2,4),4)
x <- radii[(0:15)+1]*cos((0:15)*pi/8+bearing)+loc[1]
y <- radii[(0:15)+1]*sin((0:15)*pi/8+bearing)+loc[2]
# drawing polygons
for (i in 1:15) {
x1 <- c(x[i],x[i+1],loc[1])
y1 <- c(y[i],y[i+1],loc[2])
polygon(x1,y1,col=cols[i])
}
# drawing the last polygon
polygon(c(x[16],x[1],loc[1]),c(y[16],y[1],loc[2]),col=cols[16])
# drawing letters
b <- c("E","N","W","S")
for (i in 0:3) text((size+par("cxy")[1])*cos(bearing+i*pi/2)+loc[1],
(size+par("cxy")[2])*sin(bearing+i*pi/2)+loc[2],b[i+1],
cex=cex)
}Journal of Statistical Software – Code Snippets 5
E
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Figure 2: Output of northarrow()
The argument for this function consists of the location, size, bearing, colors, and any other
graphic parameters. The size of the north arrow and the location of its center position cannot
be omitted from the functional arguments, whereas the others can be omitted. The default
north arrow is plotted in black against a white background, and it is positioned overhead (i.e.,
bearing = 0), as shown in Figure 2. Users can assign bearing for the angle (in degrees) of
the north arrow in accordance with the orientation of the map. Alternatively, users can use a
northarrow function layout.north.arrow() in sp package. The function includes two kinds
of simple arrow, but no customizable option is available.
4. Scale bar
The scale is indispensable for map users to know the magnitude of reduction and to measure
distances. Slocum et al. (2005) introduced three traditional types of scales: representative
fraction (e.g., 1:24,000), verbal scale, and scale bar (or bar scale). Among these, the scale
bar is most suited to a thematic map, particularly when distance information can enhance a
user’s understanding of the theme (Slocum et al. 2005, Chapter 11).
For an unprojected map in R, we can use the map.scale() function in the maps package. This
is applicable to a map created not only by the maps package but also by other cartographic
packages. However, in R, there is no scale bar applicable to a projected map.
We develop a scale bar function scalebar() for a projected map by using the R language.
The arguments of this function are the location, length, unit, character size of text, and any
other typical graphic parameters. Figure 3 shows a sample output of scalebar(). The design
of the scale bar is ﬁxed by this initial code. If users require other reﬁned designs, they can be
incorporated. However, we have been careful in this regard in order to maintain the clarity
of the scale (see discussion). The code for scalebar() is as follows:
scalebar <- function(loc,length,unit="km",division.cex=.8,...) {
if(missing(loc)) stop("loc is missing")
if(missing(length)) stop("length is missing")
x <- c(0,length/c(4,2,4/3,1),length*1.1)+loc[1]
y <- c(0,length/(10*3:1))+loc[2]
cols <- rep(c("black","white"),2)
for (i in 1:4) rect(x[i],y[1],x[i+1],y[2],col=cols[i])
for (i in 1:5) segments(x[i],y[2],x[i],y[3])
labels <- x[c(1,3)]-loc[1]6 Auxiliary Cartographic Functions in R
0 5000 10000 m
Figure 3: Output of scalebar() with length = 10000 and unit = ’m’
labels <- append(labels,paste(x[5]-loc[1],unit))
text(x[c(1,3,5)],y[4],labels=labels,adj=.5,cex=division.cex)
}
It should be noted that using scalebar() for a decimal degree or the entire world is not
logical because the function cannot convert units and retains them in the unconverted form
in the current graphic device. Therefore, if necessary, users must project and coordinate the
map appropriately prior to the execution of scalebar().
Users can also use a scalebar function layout.scale.bar() in the sp package. It only draws
a rectangle without text, but works well with spplot().
5. Discussion
Due to the limitations in the R graphic device, the characteristics of the label type with the
exception of type size cannot be separately arranged. However, we believe that characteristic
parameters for typography, such as fonts, letter and word spacing, kerning, and leading, that
are competently adjusted in R are acceptable even for professional use.
The rectangular box of the label is basically opaque and masks underlying objects such as
polygons or other labels in order to avoid overprint and ensure label clarity. However, if
necessary, users can use a transparent rectangular box with bgcol = "transparent".
In this paper, with regard to the“labeling”section, we propose to cull labels and use them with
a leader arrow in order to avoid congestion and overprinting. However, a more sophisticated
solution can still be expected because some systems developed specially for cartography,
such as geographic information systems (GIS), have already achieved an automated optimal
placement of labels. In the future, their algorithms can be imported to R.
The style of the north arrow should be simple so as to not attract attention. The line weights
and type must be suﬃciently ﬁne; however, we have used the default parameters (e.g., lwd)
in a graphic device. As a default setting, users draw a scale bar with solid lines that are as
thick as lines in other parts of the graphics. Optionally, users who need lines of greater or
lesser thickness can adjust the north arrow with the usual graphic parameters.
The letter representing north is ﬁxed as “N” in northarrow(). Fewer users may deal with
magnetic north abbreviated as “MN” instead of the geographical north “N”; this alternative
option has not been currently provided.
Although users can freely decide the location and the size of the north arrow, it is recom-
mended that it be relatively small in order to be inconspicuous, but yet be suﬃciently large
to ﬁnd and use. Further, it is recommended that the label be placed in a remote location,
preferably near the scale bar.
A graticule is a system of grid lines, normally representing the longitude and latitude. It is
used in a very speciﬁc case in which the north direction is variable (Slocum et al. 2005). SinceJournal of Statistical Software – Code Snippets 7
most thematic maps are projected using a coordinate system, the north arrow is suﬃcient to
indicate the orientation. Consequently, the graticule is not currently available.
The design of the scale bar obtained by scalebar() is simple and precise. Bulky, complex,
and sloppy designs must be avoided in order to ensure distinct thematic maps. The line
weights and type should be suﬃciently ﬁne, similar to the abovementioned northarrow()
function.
By using the options in scalebar(), users can also freely decide the location and length of
a scale bar on a map. However, both the location and length must be carefully selected for
ensuring that the thematic maps are clear. If possible, the scale bar should be placed below
the mapped area because map users are familiar with such an arrangement. The scale bar
must be suﬃciently long to be useful but must not occupy too much space.
When users provide the value of the length to the scalebar() function, it should always
be an integer and easily workable. For instance, when length = 7676 is assigned instead of
length = 8000, it is diﬃcult to obtain the division 7676/4. On the contrary, the quarter
division indicated in Figure 3 is deﬁnitely 2500 m. Therefore, if the maximum distance in the
scale bar is a fractional number, the clarity of the scale and map decreases.
Some scale bars include an “extension scale,” which expands the bar to the left of zero. The
scalebar() function does not support such an extension because this could confuse many
map users who expect numbers to the left of zero to be negative as in a number line.
6. Conclusion
In this study, auxiliary cartographic functions were developed in R. The labeling, north arrow,
and scale bar are expected to complement the cartographic functions in R, which will help
map users to understand the map thoroughly.
References
Lewin-Koh NJ, Bivand R (2007). maptools: Tools for Reading and Handling Spatial Objects.
R package version 0.6-8, URL http://CRAN.R-project.org/.
R Development Core Team (2007). R: A Language and Environment for Statistical Computing.
R Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0, URL http:
//www.R-project.org/.
Rowlingson B, Diggle P, Bivand R (2006). splancs: Spatial and Space–Time Point Pattern
Analysis. R package version 2.01-19, URL http://www.maths.lancs.ac.uk/~rowlings/
Splancs/.
Slocum TA, McMaster RB, Kessler FC, Howard HH (2005). Thematic Cartography and
Geographic Visualization. Pearson Education, Inc., Upper Saddle River, NJ, 2nd edition.
ISBN 0-13-035123-7.8 Auxiliary Cartographic Functions in R
Aﬃliation:
Susumu Tanimura
Department of Socio-environmental Medicine
Institute of Tropical Medicine
Nagasaki University
1-12-4 Sakamoto, Nagasaki, Japan 852-8523
E-mail: stanimura-ngs@umin.ac.jp
Journal of Statistical Software http://www.jstatsoft.org/
published by the American Statistical Association http://www.amstat.org/
Volume 19, Code Snippet 1 Submitted: 2005-12-27
April 2007 Accepted: 2007-04-10

Auxiliary Cartographic Functions in R Aﬃliation: Susumu Tanimura Department of Socio-environmental

Core Team

maptools: Tools for Reading and Handling Spatial Objects. R package version 0.6-8,

splancs: Spatial and Space–Time Point Pattern Analysis. R package version 2.01-19, URL

Thematic Cartography and Geographic Visualization. Pearson Education, Inc., Upper Saddle River, NJ, 2nd edition.

Auxilary Cartographic Functions in R: North Arrow, Scale Bar, and Label with Leader Arrow

Auxilary Cartographic Functions in R: North Arrow, Scale Bar, and Label with Leader Arrow

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