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\title{\bf Spatio-temporal overlay and aggregation }

\author{
\includegraphics[width=5cm]{ifgi-logo_int} \hspace{.5cm}
\includegraphics[width=4cm]{logo52n} \\
\href{mailto:edzer.pebesma@uni-muenster.de}{Edzer Pebesma}
}
\date{\small \today }

\begin{document}
% \VignetteIndexEntry{ Spatio-temporal overlay and aggregation }
\maketitle

\begin{abstract}
The so-called ``map overlay'' is not very well defined and does not
have a simple equivalent in space-time. This paper will explain how
the {\tt over} method for combining two spatial features (and/or
grids), defined in package {\tt sp} and extended in package {\tt
rgeos}, is implemented for spatio-temporal objects in package {\tt
spacetime}. It may carry out the numerical spatio-temporal overlay,
and can be used for aggregation of spatio-temporal data over space,
time, or space-time.
\end{abstract}

\tableofcontents

\section{Introduction}

The so-called {\em map overlay} is a key GIS operation that does not
seem to have a very sharp definition. The \code{over} vignette in
package {\tt sp} (Pebesma, 2012) comments on what paper (visual)
overlays are, and discusses the {\tt over} and {\tt aggregate}
methods for spatial data.

In the
ESRI
ArcGIS tutorial (ESRI, 2012), it can be read that
\begin{quotation}
{\em An overlay operation is much more than a simple merging of linework;
all the attributes of the features taking part in the overlay are
carried through, as shown in the example below, where parcels
(polygons) and flood zones (polygons) are overlayed (using the
Union tool) to create a new polygon layer. The parcels are split
where they are crossed by the flood zone boundary, and new polygons
created. The FID\_flood value indicates whether polygons are outside
(-1) or inside the flood zone, and all polygons retain their original
land use category values.}
\end{quotation}
It later on mentions {\em raster overlays}, such as the addition
of two (matching) raster layers (so, potentially the whole of map
algebra functions, where two layers are involved).

In the open source arena, with no budgets for English language editing, the
\href{http://grass.fbk.eu/grass70/manuals/html70_user/v.overlay.html}{Grass
7.0 documentation} mentions the following:
\begin{quotation}
{\em {\tt v.overlay} allows the user to overlay two vector area maps. The
resulting output map has a merged attribute-table. The origin
column-names have a prefix (a\_ and b\_) which results from the ainput-
and binput-map. [...] 
Operator defines features written to output vector map Feature
is written to output if the result of operation 'ainput operator
binput' is true. Input feature is considered to be true, if category
of given layer is defined.  Options: and, or, not, xor. }
\end{quotation}

\section{Overlay with method {\tt over}}

We loosely define {\em map overlay} as
\begin{itemize}
\item an operation involving at least two maps
\item asymmetric -- {\em over}lay is different from {\em under}lay
\item either a {\em visual} or a {\em numerical} activity.
\end{itemize}

The method {\tt over}, as defined in package {\tt sp}, provides
a way to numerically combine two maps. In particular,
<<eval=FALSE>>=
over(x, geometry(y))
@
returns an integer vector of length {\tt length(x)} with {\tt x[i]}
the index of {\tt y}, spatially corresponding to {\tt x[i]}, so
{\tt x[i]=j} means that {\tt x[i]} and {\tt y[j]} match (have the
same location, touch, or overlap/intersect etc.), or {\tt x[i]=NA}
if there is no match.  If {\tt y} has data values (attributes), then
<<eval=FALSE>>=
over(x, y)
@
retrieves a {\tt data.frame} with {\tt length(x)} rows, where row
{\tt i} contains the attributes of {\tt y} at the spatial location
of {\tt x[i]}, and NA values if there is no match.

If the relationship is more complex, e.g. a polygon or grid cell
{\tt x} containing more than one point of {\tt y}, the command
<<eval=FALSE>>=
over(x, y, returnList = TRUE)
@
returns a list of length {\tt length(x)}, with each list element
a numeric vector with all indices if {\tt y} is geometry only,
or else a data frame with all attribute table rows of {\tt y}
that spatially matches {\tt x[i]}.

\section{Spatio-temporal overlay with method {\tt over}}

Package {\tt spacetime} adds {\tt over} methods to those defined 
for spatial data in package {\tt sp}:
<<>>=
library(sp)
library(spacetime)
showMethods(over)
@

% <<keep.source=TRUE>>=
% download.file("http://ifgi.uni-muenster.de/~epebe_01/cw.rda", "cw.rda")
% load("cw.rda")
% unlink("cw.rda")
% @

\subsection{Time intervals or time instances?}

When computing the overlay 
<<eval=FALSE>>=
over(x, y)
@
A space-time feature matches another space-time feature when their
spatial locations match (coincide, touch, intersect or overlap),
and when their temporal properties match. For temporal properties,
it is crucial whether time is a time interval, or a time instance.
When all \code{endTime} values are equal to the \code{time} times,
time is considered instance. When one or more \code{endTime} values
are larger than \code{time}, time is considered to reflect intervals.

Suppose we have two time sequences, $T: t_1,t_2,...,t_n$ and $U:
u_1, u_2, ..., u_m$. Both are ordered: $t_i \le t_{i+1}$.

Both $T$ and $U$ can reflect time {\em instances} or time
{\em intervals}. In case they reflect time {\em instances}, an
observation at $t_i$ takes place at the time instance $t_i$, and
has an unregistered (possibly ignorable) duration. In case they
reflect time {\em intervals}, an observation ``at'' $t_i$ takes
place during, or is representatitive for, the time interval
$t_{i} \le t < t_{i+1}$. (The last time interval $t_n$ is obtained
by adopting the one-but-last time interval duration: $t_n \le t <
t_n + (t_n - t_{n-1})$).

We define the time (instance or interval) pair $\{t_i,u_j\}$ to match if
\begin{description}
\item[for $T$ instance, $U$ instance:]
$$t_i = u_j$$
\item[for $T$ interval, $U$ instance]
$$t_i \le u_j < t_{i+1}$$
\item[for $T$ instance, $U$ interval]
$$u_j \le t_i < u_{j+1}$$
\item[for $T$ interval, $U$ interval]
$$\exists t : t_i \le t < t_{i+1} \wedge u_j \le t < u_{j+1} $$
which can be rephrased as the negation of
$t_{i+1} \le u_j \vee t_i \ge u_{j+1}$ (where $\vee$ denotes `or'), 
or alternatively expressed as
$$t_{i+1} > u_j \wedge t_i < u_{j+1}$$
where $\wedge$ denotes `and'.
\end{description}
All these conditions fail for intervals having zero width (empty
intervals), i.e. the case where $T$ is interval and for some $i$,
$t_{i+1}-t_i=0$ or the case where $U$ is interval and for some $j$,
$u_{j+1}-u_j=0$.


\section{Aggregating spatio-temporal data}

The {\tt aggregate} method for a {\tt data.frame} is defined as
<<eval=FALSE>>=
aggregate(x, by, FUN, ..., simplify = TRUE)
@
where {\tt x} is the {\tt data.frame} to be aggregated, {\tt by}
indicates how groups of {\tt x} are formed, {\tt FUN} is applied to
each group, and {\tt simplify} indicates whether the output should
be simplified (to vector), or remain a {\tt data.frame}. The {\tt
...} are passed to {\tt FUN}, e.g. passing {\tt na.rm=TRUE} is useful
when {\tt FUN} is {\tt mean} and missing values need to be ignored.

For spatio-temporal data, the {\tt x} argument needs to be of class
{\tt STFDF}, {\tt STSDF} or {\tt STIDF}. The {\tt by} argument
needs to specify an aggregation medium: time, space, or space-time.

\subsection{Example data: PM10}

Air quality example data are loaded by
<<>>=
data(air)
rural = STFDF(stations, dates, data.frame(PM10 = as.vector(air)))
class(rural)
class(DE_NUTS1)
@
it provides PM10 daily mean values (taken from
\href{http://acm.eionet.europa.eu/databases/airbase}{AirBase -
the European Air quality dataBase}), for Germany, 1998-2009, where
only stations classified as {\em rural background} were selected.
The object {\tt DE\_NUTS1} contains NUTS-1 level state boundaries
for Germany, downloaded from \href{http://www.gadm.org/}{GADM}.

\subsection{Spatial aggregation}

To aggregate {\em completely} over space, we can coerce the
data to a matrix and apply a function to the rows:
<<>>=
x = as(rural[,"2008"], "xts")
apply(x, 1, mean, na.rm=TRUE)[1:5]
@

A more refined spatial aggregation of time series can be obtained by grouping
them to the state (``Bundesland'') level. Here, states are passed as
a {\tt SpatialPolygons} object:
<<>>=
dim(rural[,"2008"])
x = aggregate(rural[,"2008"], DE_NUTS1, mean, na.rm=TRUE)
dim(x)
summary(x)
stplot(x, mode = "tp")
@
the result of which is shown in figure \ref{fig:tp1}, which was
created by
<<eval=FALSE>>=
stplot(x, mode = "tp", par.strip.text = list(cex=.5))
@

\begin{figure}
<<fig=TRUE,echo=FALSE>>=
print(stplot(x, mode = "tp", par.strip.text = list(cex=.5)))
@
\caption{Daily PM10 values, aggregated (averaged) over states}
\label{fig:tp1}
\end{figure}

An aggregation for all stations selected within a single
area is obtained by using the country boundary \code{DE},
and aggregating the observations within Germany for
each moment in time:
<<>>=
x = aggregate(rural[,"2008"], DE, mean, na.rm=TRUE)
class(x)
plot(x[,"PM10"])
@
the plot of which is shown in figure \ref{fig:x}.

\begin{figure}
<<fig=TRUE,echo=FALSE>>=
plot(x[,"PM10"])
@
\caption{Time series plot of daily rural background PM10, averaged
over Germany}
\label{fig:x}
\end{figure}

\subsection{Temporal aggregation}

To aggregate {\em completely} over time, we can coerce the
data to a matrix and apply a function to the columns:
<<>>=
x = as(rural[,"2008"], "xts")
apply(x, 2, mean, na.rm=TRUE)[1:5]
@

Aggregating values {\em temporally} is done by passing
a character string or a function to the {\tt by} argument.
For monthly data, we will first select those stations that
have measured (non-NA) values in 2008,
<<>>=
sel = which(!apply(as(rural[,"2008"], "xts"), 2, function(x) all(is.na(x))))
x = aggregate(rural[sel,"2008"], "month", mean, na.rm=TRUE)
stplot(x, mode = "tp")
@
shown in figure \ref{fig:tp2}

\begin{figure}
<<fig=TRUE,echo=FALSE>>=
print(stplot(x, mode = "tp", par.strip.text = list(cex=.5)))
@
\caption{ Monthly averaged PM10 values, for those rural background
stations in Germany having measured values }
\label{fig:tp2}
\end{figure}

The strings that can be passed are e.g. {\tt "year"}, but also {\tt
"3 days"}. See {\tt ?cut.Date} for possible values. Aggregation using
this way is only possible if the time index is of class {\tt Date}
or {\tt POSIXct}.

An alternative is to provide a function for temporal aggregation. The
function \code{as.yearqtr} from package \code{zoo} transforms dates
to quarters, and hence allows aggregation {\em to} quarterly values,
in this example medians:
<<>>=
library(zoo)
x = aggregate(rural[sel,"2005::2011"], as.yearqtr, median, na.rm=TRUE)
stplot(x, mode = "tp")
@
shown in figure \ref{fig:tp3}. Aggregating to monthly values is obtained
by function {\code as.yearmon}, aggregating to years by creating the
function
<<>>=
as.year <- function(x) as.numeric(floor(as.yearmon(x)))
@
Further information can be found in {\tt ?aggregate.zoo}, which is
the function used to do the processing.

\begin{figure}
<<fig=TRUE,echo=FALSE>>=
print(stplot(x, mode = "tp", par.strip.text = list(cex=.5)))
@
\caption{PM10 values, averaged to quarterly medians of daily averages}
\label{fig:tp3}
\end{figure}

\subsection{Spatio-temporal aggregation}

Aggregation over spatio-temporal volumes can be done by passing
an object inheriting from {\tt ST} to the {\tt by} argument:
<<>>=
DE.years = STF(DE, as.Date(c("2008-01-01", "2009-01-01")))
aggregate(rural[,"2008::2009"], DE.years, mean, na.rm=TRUE)
@

\subsection{Time intervals}

If all data concern time instances (endTime equals time), then time
instances are being matched, else overlapping time intervals are
being matched. In case intervals are being matched, empty intervals
are never matched.

\subsection*{References}
\begin{itemize}
\item ESRI (2012) ESRI ArcGIS Tutorial. \url{http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?TopicName=Overlay_analysis}
\item Pebesma, 2012. Map overlay and
spatial aggregation in sp. Vignette in package sp,
\url{http://cran.r-project.org/web/packages/sp/vignettes/over.pdf}
\end{itemize}

\end{document}
