3  Geometries

Having learned how we represent coordinates systems, we can define how geometries can be described using these coordinate systems. This chapter will explain:

Geometries on the sphere are discussed in Chapter 4, rasters and other rectangular subdivisions of space are discussed in Chapter 6.

3.1 Simple feature geometries

Simple feature geometries are a way to describe the geometries of features. By features we mean things that have a geometry, potentially implicitly some time properties, and further attributes that could include labels describing the thing and/or values quantitatively measuring it. The main application of simple feature geometries is to describe geometries in two-dimensional space by points, lines, or polygons. The “simple” adjective refers to the fact that the line or polygon geometries are represented by sequences of points connected with straight lines that do not self-intersect.

Simple features access is a standard (Herring 2011, 2010; ISO 2004) for describing simple feature geometries that includes:

  • a class hierarchy
  • a set of operations
  • binary and text encodings

We will first discuss the seven most common simple feature geometry types.

The big seven

The most commonly used simple features geometries, used to represent a single feature are:

type description
POINT single point geometry
MULTIPOINT set of points
LINESTRING single linestring (two or more points connected by straight lines)
MULTILINESTRING set of linestrings
POLYGON exterior ring with zero or more inner rings, denoting holes
MULTIPOLYGON set of polygons
GEOMETRYCOLLECTION set of the geometries above
Code
library(sf) |> suppressPackageStartupMessages()
par(mfrow = c(2,4))
par(mar = c(1,1,1.2,1))

# 1
p <- st_point(0:1)
plot(p, pch = 16)
title("point")
box(col = 'grey')

# 2
mp <- st_multipoint(rbind(c(1,1), c(2, 2), c(4, 1), c(2, 3), c(1,4)))
plot(mp, pch = 16)
title("multipoint")
box(col = 'grey')

# 3
ls <- st_linestring(rbind(c(1,1), c(5,5), c(5, 6), c(4, 6), c(3, 4), c(2, 3)))
plot(ls, lwd = 2)
title("linestring")
box(col = 'grey')

# 4
mls <- st_multilinestring(list(
  rbind(c(1,1), c(5,5), c(5, 6), c(4, 6), c(3, 4), c(2, 3)),
  rbind(c(3,0), c(4,1), c(2,1))))
plot(mls, lwd = 2)
title("multilinestring")
box(col = 'grey')

# 5 polygon
po <- st_polygon(list(rbind(c(2,1), c(3,1), c(5,2), c(6,3), c(5,3), c(4,4), c(3,4), c(1,3), c(2,1)),
    rbind(c(2,2), c(3,3), c(4,3), c(4,2), c(2,2))))
plot(po, border = 'black', col = '#ff8888', lwd = 2)
title("polygon")
box(col = 'grey')

# 6 multipolygon
mpo <- st_multipolygon(list(
    list(rbind(c(2,1), c(3,1), c(5,2), c(6,3), c(5,3), c(4,4), c(3,4), c(1,3), c(2,1)),
        rbind(c(2,2), c(3,3), c(4,3), c(4,2), c(2,2))),
    list(rbind(c(3,7), c(4,7), c(5,8), c(3,9), c(2,8), c(3,7)))))
plot(mpo, border = 'black', col = '#ff8888', lwd = 2)
title("multipolygon")
box(col = 'grey')

# 7 geometrycollection
gc <- st_geometrycollection(list(po, ls + c(0,5), st_point(c(2,5)), st_point(c(5,4))))
plot(gc, border = 'black', col = '#ff6666', pch = 16, lwd = 2)
title("geometrycollection")
box(col = 'grey')

Figure 3.1: sketches of the main simple feature geometry types

Figure 3.1 shows examples of these basic geometry types. The human-readable, “well-known-text” (WKT) representation of the geometries plotted are:

Code
p
mp
ls
mls
po
mpo
gc
POINT (0 1)
MULTIPOINT ((1 1), (2 2), (4 1), (2 3), (1 4))
LINESTRING (1 1, 5 5, 5 6, 4 6, 3 4, 2 3)
MULTILINESTRING ((1 1, 5 5, 5 6, 4 6, 3 4, 2 3), (3 0, 4 1, 2 1))
POLYGON ((2 1, 3 1, 5 2, 6 3, 5 3, 4 4, 3 4, 1 3, 2 1),
    (2 2, 3 3, 4 3, 4 2, 2 2))
MULTIPOLYGON (((2 1, 3 1, 5 2, 6 3, 5 3, 4 4, 3 4, 1 3, 2 1),
    (2 2, 3 3, 4 3, 4 2, 2 2)), ((3 7, 4 7, 5 8, 3 9, 2 8, 3 7)))
GEOMETRYCOLLECTION (
    POLYGON ((2 1, 3 1, 5 2, 6 3, 5 3, 4 4, 3 4, 1 3, 2 1),
      (2 2 , 3 3, 4 3, 4 2, 2 2)),
    LINESTRING (1 6, 5 10, 5 11, 4 11, 3 9, 2 8),
    POINT (2 5),
    POINT (5 4)
)

In this representation, coordinates are separated by space, and points by commas. Sets are grouped by parentheses, and separated by commas. Polygons consist of an outer ring followed by zero or more inner rings denoting holes.

Individual points in a geometry contain at least two coordinates: \(x\) and \(y\), in that order. If these coordinates refer to ellipsoidal coordinates, \(x\) and \(y\) usually refer to longitude and latitude, respectively, although sometimes to latitude and longitude (see Section 2.4 and Section 7.7.6).

Simple and valid geometries, ring direction

Linestrings are called simple when they do not self-intersect:

Code
(ls <- st_linestring(rbind(c(0,0), c(1,1), c(2,2), c(0,2), c(1,1), c(2,0))))
# LINESTRING (0 0, 1 1, 2 2, 0 2, 1 1, 2 0)
c(is_simple = st_is_simple(ls))
# is_simple 
#     FALSE

Valid polygons and multipolygons obey all of the following properties:

  • polygon rings are closed (the last point equals the first)
  • polygon holes (inner rings) are inside their exterior ring
  • polygon inner rings maximally touch the exterior ring in single points, not over a line
  • a polygon ring does not repeat its own path
  • in a multipolygon, an external ring maximally touches another exterior ring in single points, not over a line

If this is not the case, the geometry concerned is not valid. Invalid geometries typically cause errors when they are processed, but can usually be repaired to make them valid.

A further convention is that the outer ring of a polygon is winded counter-clockwise, while the holes are winded clockwise, but polygons for which this is not the case are still considered valid. For polygons on the sphere, the “clockwise” concept is not very useful: if for instance we take the equator as polygon, is the Northern hemisphere or the Southern hemisphere “inside”? The convention taken here is to consider the area on the left while traversing the polygon is considered the polygon’s inside.

Z and M coordinates

In addition to X and Y coordinates, Single points (vertices) of simple feature geometries may have:

  • a Z coordinate, denoting altitude, and/or
  • an M value, denoting some “measure”

The M attribute shall be a property of the vertex. It sounds attractive to encode a time stamp in it, e.g. to pack movement data (trajectories) in LINESTRINGs. These become however invalid (or “non-simple”) once the trajectory self-intersects, which easily happens when only X and Y are considered for self-intersections.

Both Z and M are not found often, and software support to do something useful with them is (still) rare. Their WKT representation are fairly easily understood:

Code
st_point(c(1,3,2))
# POINT Z (1 3 2)
st_point(c(1,3,2), dim = "XYM")
# POINT M (1 3 2)
st_linestring(rbind(c(3,1,2,4), c(4,4,2,2)))
# LINESTRING ZM (3 1 2 4, 4 4 2 2)

Empty geometries

A very important concept in the feature geometry framework is that of the empty geometry. Empty geometries arise naturally when we do geometrical operations (Section 3.2), for instance when we want to know the intersection of POINT (0 0) and POINT (1 1):

Code
(e <- st_intersection(st_point(c(0,0)), st_point(c(1,1))))
# GEOMETRYCOLLECTION EMPTY

and it represents essentially the empty set: when combining (unioning) an empty point it with other non-empty geometries, it vanishes.

All geometry types have a special value representing the empty (typed) geometry:

Code
st_point()
# POINT EMPTY
st_linestring(matrix(1,1,3)[0,], dim = "XYM")
# LINESTRING M EMPTY

and so on, but they all point to the empty set, differing only in their dimension (Section 3.2.2).

Ten further geometry types

There are 10 more geometry types which are more rare, but increasingly find implementation:

type description
CIRCULARSTRING The CIRCULARSTRING is the basic curve type, similar to a LINESTRING in the linear world. A single segment requires three points, the start and end points (first and third) and any other point on the arc. The exception to this is for a closed circle, where the start and end points are the same. In this case the second point MUST be the center of the arc, i.e. the opposite side of the circle. To chain arcs together, the last point of the previous arc becomes the first point of the next arc, just like in LINESTRING. This means that a valid circular string must have an odd number of points greater than 1.
COMPOUNDCURVE A compound curve is a single, continuous curve that has both curved (circular) segments and linear segments. That means that in addition to having well-formed components, the end point of every component (except the last) must be coincident with the start point of the following component.
CURVEPOLYGON Example compound curve in a curve polygon: CURVEPOLYGON(COMPOUNDCURVE(CIRCULARSTRING(0 0,2 0, 2 1, 2 3, 4 3),(4 3, 4 5, 1 4, 0 0)), CIRCULARSTRING(1.7 1, 1.4 0.4, 1.6 0.4, 1.6 0.5, 1.7 1) )
MULTICURVE A MultiCurve is a 1-dimensional GeometryCollection whose elements are Curves, it can include linear strings, circular strings or compound strings.
MULTISURFACE A MultiSurface is a 2-dimensional GeometryCollection whose elements are Surfaces, all using coordinates from the same coordinate reference system.
CURVE A Curve is a 1-dimensional geometric object usually stored as a sequence of Points, with the subtype of Curve specifying the form of the interpolation between Points
SURFACE A Surface is a 2-dimensional geometric object
POLYHEDRALSURFACE A PolyhedralSurface is a contiguous collection of polygons, which share common boundary segments
TIN A TIN (triangulated irregular network) is a PolyhedralSurface consisting only of Triangle patches.
TRIANGLE A Triangle is a polygon with 3 distinct, non-collinear vertices and no interior boundary

CIRCULARSTRING, COMPOUNDCURVE and CURVEPOLYGON are not described in the SFA standard, but in the SQL-MM part 3 standard. The descriptions above were copied from the PostGIS manual.

Text and binary encodings

Part of the simple feature standard are two encodings: a text and a binary encoding. The well-known text encoding, used above, is human-readable, the well-known binary encoding is machine-readable. Well-known binary (WKB) encodings are lossless and typically faster to work with than text encoding (and decoding), and are used for instance in all communications between R package sf and the GDAL, GEOS, liblwgeom and s2geometry libraries (Figure 1.7).

3.2 Operations on geometries

Simple feature geometries can be queried for properties, transformed into new geometries, and combinations of geometries can be queried for properties. This section gives an overview of the operations entirely focusing on geometrical properties. Chapter 5 focuses on the analysis of non-geometrical feature properties, in relationship to their geometries. Some of the material in this section appeared in Pebesma (2018).

We can categorize operations on geometries in terms of what they take as input, and what they return as output. In terms of output we have operations that return:

  • predicates: a logical asserting a certain property is TRUE
  • measures: a quantity (e.g. a numeric value with measurement unit)
  • transformations: newly generated geometries

and in terms of what they operate on, we distinguish operations that are:

  • unary when they work on a single geometry
  • binary when they work on pairs of geometries
  • n-ary when they work on sets of geometries

Unary predicates

Unary predicates describe a certain property of a geometry. The predicates is_simple, is_valid, and is_empty return respectively whether a geometry is simple, valid or empty. Given a coordinate reference system, is_longlat returns whether the coordinates are geographic or projected. is(geometry, class) checks whether a geometry belongs to a particular class.

Binary predicates and DE-9IM

The Dimensionally Extended Nine-Intersection Model (DE-9IM, Clementini, Di Felice, and Oosterom (1993); Egenhofer and Franzosa (1991)) is a model that helps describing the qualitative relation between any two geometries in two-dimensional space (\(R^2\)). Any geometry has a dimension value that is:

  • 0 for points,
  • 1 for linear geometries,
  • 2 for polygonal geometries, and
  • F (false) for empty geometries

Any geometry also has an inside (I), a boundary (B) and an exterior (E); these roles are obvious for polygons but, e.g. for:

  • lines the boundary is formed by the end points, and the interior by all non-end points on the line
  • points have a zero-dimensional inside but no boundary
Code
library(sf)
polygon <- po <- st_polygon(list(rbind(c(0,0), c(1,0), c(1,1), c(0,1), c(0,0))))
p0 <- st_polygon(list(rbind(c(-1,-1), c(2,-1), c(2,2), c(-1,2), c(-1,-1))))
line <- li <- st_linestring(rbind(c(.5, -.5), c(.5, 0.5)))
s <- st_sfc(po, li)

par(mfrow = c(3,3))
par(mar = c(1,1,1,1))

# "1020F1102"
# 1: 1
plot(s, col = c(NA, 'darkgreen'), border = 'blue', main = expression(paste("I(pol)",intersect(),"I(line) = 1")))
lines(rbind(c(.5,0), c(.5,.495)), col = 'red', lwd = 2)
points(0.5, 0.5, pch = 1)

# 2: 0
plot(s, col = c(NA, 'darkgreen'), border = 'blue', main = expression(paste("I(pol)",intersect(),"B(line) = 0")))
points(0.5, 0.5, col = 'red', pch = 16)

# 3: 2
plot(s, col = c(NA, 'darkgreen'), border = 'blue', main = expression(paste("I(pol)",intersect(),"E(line) = 2")))
plot(po, col = '#ff8888', add = TRUE)
plot(s, col = c(NA, 'darkgreen'), border = 'blue', add = TRUE)

# 4: 0
plot(s, col = c(NA, 'darkgreen'), border = 'blue', main = expression(paste("B(pol)",intersect(),"I(line) = 0")))
points(.5, 0, col = 'red', pch = 16)

# 5: F
plot(s, col = c(NA, 'darkgreen'), border = 'blue', main = expression(paste("B(pol)",intersect(),"B(line) = F")))

# 6: 1
plot(s, col = c(NA, 'darkgreen'), border = 'blue', main = expression(paste("B(pol)",intersect(),"E(line) = 1")))
plot(po, border = 'red', col = NA, add = TRUE, lwd = 2)

# 7: 1
plot(s, col = c(NA, 'darkgreen'), border = 'blue', main = expression(paste("E(pol)",intersect(),"I(line) = 1")))
lines(rbind(c(.5, -.5), c(.5, 0)), col = 'red', lwd = 2)

# 8: 0
plot(s, col = c(NA, 'darkgreen'), border = 'blue', main = expression(paste("E(pol)",intersect(),"B(line) = 0")))
points(.5, -.5, col = 'red', pch = 16)

# 9: 2
plot(s, col = c(NA, 'darkgreen'), border = 'blue', main = expression(paste("E(pol)",intersect(),"E(line) = 2")))
plot(p0 / po, col = '#ff8888', add = TRUE)
plot(s, col = c(NA, 'darkgreen'), border = 'blue', add = TRUE)

Figure 3.2: DE-9IM: intersections between the interior, boundary and exterior of a polygon (rows) and of a linestring (columns) indicated by red

Figure 3.2 shows the intersections between the I, B and E components of a polygon and a linestring indicated by red; the sub-plot title gives the dimension of these intersections (0, 1, 2 or F). The relationship between the polygon and the line geometry is the concatenation of these dimensions:

Code
st_relate(polygon, line)
#      [,1]       
# [1,] "1020F1102"

where the first three characters are associated with the inside of the first geometry (the polygon): Figure 3.2 is summarised row-wise. Using this ability to express relationships, we can also query pairs of geometries about particular conditions expressed in a mask string; e.g. the string "*0*******" would evaluate TRUE when the second geometry has one or more boundary points in common with the interior of the first geometry; the symbol * standing for “any dimensionality” (0, 1, 2 or F). The mask string "T********" matches pairs of geometry with intersecting interiors, where the symbol T stands for any non-empty intersection of dimensionality 0, 1 or 2.

Binary predicates are further described using normal-language verbs, using DE-9IM definitions. For instance, the predicate equals corresponds to the relationship "T*F**FFF*". If any two geometries obey this relationship, they are (topologically) equal, but may have a different ordering of nodes.

A list of binary predicates is:

predicate meaning inverse of
contains None of the points of A are outside B within
contains_properly A contains B and B has no points in common with the boundary of A
covers No points of B lie in the exterior of A covered_by
covered_by Inverse of covers
crosses A and B have some but not all interior points in common
disjoint A and B have no points in common intersects
equals A and B are topologically equal: node order or number of nodes may differ; identical to A contains B AND A within B
equals_exact A and B are geometrically equal, and have identical node order
intersects A and B are not disjoint disjoint
is_within_distance A is closer to B than a given distance
within None of the points of B are outside A contains
touches A and B have at least one boundary point in common, but no interior points
overlaps A and B have some points in common; the dimension of these is identical to that of A and B
relate given a mask pattern, return whether A and B adhere to this pattern

The Wikipedia DE-9IM page provides the relate patterns for each of these verbs. They are important to check out; for instance covers and contains (and their inverses) are often not completely intuitive:

  • if A contains B, B has no points in common with the exterior or boundary of A
  • if A covers B, B has no points in common with the exterior of A

Unary Measures

Unary measures return a measure or quantity that describes a property of the geometry:

measure returns
dimension 0 for points, 1 for linear, 2 for polygons, possibly NA for empty geometries
area the area of a geometry
length the length of a linear geometry

Binary Measures

distance returns the distance between pairs of geometries. The qualitative measure relate (without mask) gives the relation pattern, a description of the geometrical relationship between two geometries explained in Section 3.2.2.

Unary Transformers

Unary transformations work on a per-geometry basis, and for each geometry return a new geometry.

transformer returns a geometry …
centroid of type POINT with the geometry’s centroid
buffer that is this larger (or smaller) than the input geometry, depending on the buffer size
jitter that was moved in space a certain amount, using a bivariate uniform distribution
wrap_dateline cut into pieces that do no longer cover or cross the dateline
boundary with the boundary of the input geometry
convex_hull that forms the convex hull of the input geometry (Figure 3.3)
line_merge after merging connecting LINESTRING elements of a MULTILINESTRING into longer LINESTRINGs.
make_valid that is valid
node with added nodes to linear geometries at intersections without a node; only works on individual linear geometries
point_on_surface with a (arbitrary) point on a surface
polygonize of type polygon, created from lines that form a closed ring
segmentize a (linear) geometry with nodes at a given density or minimal distance
simplify simplified by removing vertices/nodes (lines or polygons)
split that has been split with a splitting linestring
transform transformed or convert to a new coordinate reference system (Chapter 2)
triangulate with Delauney triangulated polygon(s) (Figure 3.3)
voronoi with the Voronoi tessellation of an input geometry (Figure 3.3)
zm with removed or added Z and/or M coordinates
collection_extract with subgeometries from a GEOMETRYCOLLECTION of a particular type
cast that is converted to another type
+ that is shifted over a given vector
* that is multiplied by a scalar or matrix
Code
par(mar = rep(0,4), mfrow = c(1, 3))
set.seed(133331)
mp <- st_multipoint(matrix(runif(20), 10))
plot(mp, cex = 2)
plot(st_convex_hull(mp), add = TRUE, col = NA, border = 'red')
box()
plot(mp, cex = 2)
plot(st_voronoi(mp), add = TRUE, col = NA, border = 'red')
box()
plot(mp, cex = 2)
plot(st_triangulate(mp), add = TRUE, col = NA, border = 'darkgreen')
box()

Figure 3.3: for a set of points, left: convex hull (red); middle: Voronoi polygons; right: Delauney triangulation

Binary Transformers

Binary transformers are functions that return a geometry based on operating on a pair of geometries. They include:

function returns infix operator
intersection the overlapping geometries for pair of geometries &
union the combination of the geometries; removes internal boundaries and duplicate points, nodes or line pieces |
difference the geometries of the first after removing the overlap with the second geometry /
sym_difference the combinations of the geometries after removing where they intersect; the negation (opposite) of intersection %/%

N-ary Transformers

N-ary transformers operate on sets of geometries. union can be applied to a set of geometries to return its geometrical union. Otherwise, any set of geometries can be combined into a MULTI-type geometry when they have equal dimension, or else into a GEOMETRYCOLLECTION. Without unioning, this may lead to a geometry that is not valid, e.g. because two polygon rings have a boundary line in common.

N-ary intersection and difference take a single argument, but operate (sequentially) on all pairs, triples, quadruples, etc. Consider the plot in Figure 3.4 : how do we identify the area where all three boxes overlap? Using binary intersections gives us intersections for all pairs: 1-1, 1-2, 1-3, 2-1, 2-2, 2-3, 3-1, 3-2, 3-3, but does not let us identify areas where more than two geometries intersect. Figure 3.4) (right shows the n-ary intersection: the 7 unique, non-overlapping geometries originating from intersection of one, two, or more geometries.

Code
par(mar = rep(.1, 4), mfrow = c(1, 2))
sq <- function(pt, sz = 1) st_polygon(list(rbind(c(pt - sz), 
  c(pt[1] + sz, pt[2] - sz), c(pt + sz), c(pt[1] - sz, pt[2] + sz), c(pt - sz))))
x <- st_sf(box = 1:3, st_sfc(sq(c(0,0)), sq(c(1.7, -0.5)), sq(c(0.5, 1))))
plot(st_geometry(x), col = NA, border = sf.colors(3, categorical = TRUE), lwd = 3)
plot(st_intersection(st_geometry(x)), col = sf.colors(7, categorical=TRUE, alpha = .5))

Figure 3.4: left: three overlapping boxes – how do we identify the small box where all three overlap? right: unique, non-overlapping n-ary intersections

Similarly, one can compute an n-ary difference from a set \(\{s_1, s_2, s_3, ...\}\) by creating differences \(\{s_1, s_2-s_1, s_3-s_2-s_1, ...\}\). This is shown in Figure 3.5 , left for the original set, right for the set after reversing its order to make clear that the result here depends on the ordering of the input geometries. Again, resulting geometries do not overlap.

Code
par(mar = rep(.1, 4), mfrow = c(1, 2)) 
xg <- st_geometry(x)
plot(st_difference(xg), col = sf.colors(3, alpha = .5, categorical=TRUE))
plot(st_difference(xg[3:1]), col = sf.colors(3, alpha = .5, categorical=TRUE))

Figure 3.5: difference between subsequent boxes, left: in original order; right: in reverse order

3.3 Precision

Geometrical operations, such as finding out whether a certain point is on a line, may fail when coordinates are represented by double precision floating point numbers, such as 8-byte doubles used in R. An often chosen remedy is to limit the precision of the coordinates before the operation. For this, a precision model is adopted; the most common is to choose a factor \(p\) and compute rounded coordinates \(c'\) from original coordinates \(c\) by \[c' = \mbox{round}(p \cdot c) / p\]

Rounding of this kind brings the coordinates to points on a regular grid with spacing \(1/p\), which is beneficial for geometric computations. Of course, it also affects all computations like areas and distances, and may turn valid geometries into invalid ones. Which precision values are best for which application is often a matter of common sense combined with trial and error.

3.4 Coverages: tessellations and rasters

The Open Geospatial Consortium defines a coverage as a “feature that acts as a function to return values from its range for any direct position within its spatiotemporal domain” (Baumann, Hirschorn, and Masó 2017). Having a function implies that for every “point”, i.e. every combination of spatial point and a moment in time of the spatiotemporal domain, we have single value for the range. This is a very common situation for spatiotemporal phenomena, a few examples can be given:

  • boundary disputes aside, every point in a region (domain) belongs to a single administrative unit (range)
  • at any given moment in time, every point in a region (domain) has a certain land cover type (range)
  • every point in an area (domain) has a single elevation (range), e.g. measured with respect to a given mean sea level surface
  • every spatiotemporal point in a three-dimensional body of air (domain) has single value for temperature (range)

A caveat here is that because observation or measurement always takes time and requires space, measured values are always an average over a spatiotemporal volume, and hence range variables can rarely be measured for true, zero-volume “points”; for many practical cases however the measured volume is small enough to be considered a “point”; for a variable like land cover type the volume needs to be chosen such that the types distinguished make sense with respect to the measured units.

In the first two of the given examples the range variable is categorical, in the last two the range variable is continuous. For categorical range variables, if large connected areas have a constant range value, an efficient way to represent these data is by storing the boundaries of the areas with constant value, such as country boundaries. Although this can be done (and is often done) by a set of simple feature geometries (polygons or multipolygons), this brings along some challenges:

  • it is hard to guarantee for such a set of simple feature polygons that they do not overlap, or that there are no unwanted gaps between them
  • simple features have no way of assigning points on the boundary of two adjacent polygons uniquely to a single polygon, which conflicts with the interpretation as coverage

Topological models

A data model that guarantees no inadvertent gaps or overlaps of polygonal coverages is the topological model, examples of which are found in geographic information systems (GIS) like GRASS GIS or ArcGIS. Topological models store boundaries between polygons only once, and register which polygonal area is on either side of a boundary.

Deriving the set of (multi)polygons for each area with a constant range value from a topological model is straightforward; the other way around: reconstructing topology from a set of polygons typically involves setting thresholds on errors and handling gaps or overlaps.

Raster tessellations

A tessellation is a subdivision of a space (area, volume) into smaller elements by ways of polygons. A regular tessellation does this with regular polygons: triangles, squares or hexagons. Tessellations using squares are commonly used for spatial data, and are called raster data. Raster data tessellate each spatial dimension \(d\) into regular cells, formed e.g. by left-closed and right-open intervals \(d_i\): \[\begin{equation} d_i = d_0 + [i \times \delta, (i+1) \times \delta) \end{equation}\] with \(d_0\) an offset, \(\delta\) the interval (cell or pixel) size, and where the cell index \(i\) is an arbitrary but consecutive set of integers. The \(\delta\) value is often taken negative for the \(y\)-axis (Northing), indicating that raster row numbers increasing Southwards correspond to \(y\)-coordinates increasing Northwards.

Where in arbitrary polygon tessellations the assignment of points to polygons is ambiguous for points falling on a boundary shared by two polygons, using left-closed “[” and right-open “)” intervals in regular tessellations removes this ambiguity. This means that for rasters with negative \(\delta\) values for the \(y\)-coordinate and positive for the \(x\)-coordinate, only the top-left corner point is part of each raster cell. An artifact resulting from this is shown in Figure 3.6 .

Code
library(stars) |> suppressPackageStartupMessages()
par(mar = rep(1, 4))
ls <- st_sf(a = 2, st_sfc(st_linestring(rbind(c(0.1, 0), c(1, .9)))))
grd <- st_as_stars(st_bbox(ls), nx = 10, ny = 10, xlim = c(0, 1.0), ylim = c(0, 1),
   values = -1)
r <- st_rasterize(ls, grd, options = "ALL_TOUCHED=TRUE")
r[r == -1] <- NA
plot(st_geometry(st_as_sf(grd)), border = 'orange', col = NA, 
     reset = FALSE, key.pos = NULL)
plot(r, axes = FALSE, add = TRUE, breaks = "equal", main = NA) # ALL_TOUCHED=FALSE;
plot(ls, add = TRUE, col = "red", lwd = 2)

Figure 3.6: Rasterization artifact: as only top-left corners are part of the raster cell, only cells touching the red line below the diagonal line are rasterized

Tessellating the time dimension with left-closed, right-open intervals is very common, and reflects the implicit assumption underlying time series software such as the xts package in R, where time stamps indicate the start of time intervals. Different models can be combined: one could use simple feature polygons to tessellate space, and combine this with a regular tessellation of time in order to cover a space-time vector data cube. Raster and vector data cubes are discussed in Chapter 6.

As mentioned above, besides square cells the other two shapes that can lead to regular tessellations of \(R^2\) are triangles and hexagons. On the sphere, there are a few more, including cube, octahedron, icosahedron and dodecahedron. A spatial index that builds on the cube is s2geometry, the H3 library uses the icosahedron and densifies that with (mostly) hexagons. Mosaics that cover the entire Earth are also called discrete global grids.

3.5 Networks

Spatial networks are typically composed of linear (LINESTRING) elements, but possess further topological properties describing the network coherence:

  • start and endpoints of a linestring may be connected to other linestring start or end points, forming a set of nodes and edges
  • edges may be directed, to only allow for connection (flow, transport) in one way

R packages including osmar (Schlesinger and Eugster 2013), stplanr (Lovelace, Ellison, and Morgan 2022) and sfnetworks (van der Meer et al. 2022) provide functionality for constructing network objects, and working with them, e.g. computing shortest or fastest routes through a network. Package spatstat (Baddeley, Turner, and Rubak 2022; Baddeley, Rubak, and Turner 2015) has infrastructure for analysing point patterns on linear networks (Chapter 11). Chapter 12 of Lovelace, Nowosad, and Muenchow (2019) has a transportation application using networks.

3.6 Exercises

For the following exercises, use R where possible.

  1. Give two examples of geometries in 2-D (flat) space that cannot be represented as simple feature geometries, and create a plot of them.
  2. Recompute the coordinates 10.542, 0.01, 45321.6789 using precision values 1, 1e3, 1e6, and 1e-2.
  3. Describe a practical problem for which an n-ary intersection would be needed.
  4. How can you create a Voronoi diagram (Figure 3.3) that has one closed polygons for every single point?
  5. Give the unary measure dimension for geometries POINT Z (0 1 1), LINESTRING Z (0 0 1,1 1 2), and POLYGON Z ((0 0 0,1 0 0,1 1 0,0 0 0))
  6. Give the DE-9IM relation between LINESTRING(0 0,1 0) and LINESTRING(0.5 0,0.5 1); explain the individual characters.
  7. Can a set of simple feature polygons form a coverage? If so, under which constraints?
  8. For the nc counties in the dataset that comes with R package sf, find the points touched by four counties.
  9. How would Figure 3.6 look like if \(\delta\) for the \(y\)-coordinate was positive?