Documentation

Intersection of two latitude-longitude quadrangles

Syntax

[latlim, lonlim] = intersectgeoquad(latlim1, lonlim1, latlim2, lonlim2)

Description

[latlim, lonlim] = intersectgeoquad(latlim1, lonlim1, latlim2, lonlim2) computes the intersection of the quadrangle defined by the latitude and longitude limits latlim1 and lonlim1, with the quadrangle defined by the latitude and longitude limits latlim2 and lonlim2. latlim1 and latlim2 are two-element vectors of the form [southern-limit northern-limit]. Likewise, lonlim1 and lonlim2 are two-element vectors of the form [western-limit eastern-limit]. All input and output angles are in units of degrees. The intersection results are given in the output arrays latlim and lonlim. Given an arbitrary pair of input quadrangles, there are three possible results:

1. The quadrangles fail to intersect. In this case, both latlim and lonlim are empty arrays.

2. The intersection consists of a single quadrangle. In this case, latlim (like latlim1 and latlim2) is a two-element vector that has the form [southern-limit northern-limit], where southern-limit and northern-limit represent scalar values. lonlim (like lonlim1 and lonlim2), is a two-element vector that has the form [western-limit eastern-limit], with a pair of scalar limits.

3. The intersection consists of a pair of quadrangles. This can happen when longitudes wrap around such that the eastern end of one quadrangle overlaps the western end of the other and vice versa. For example, if lonlim1 = [-90 90] and lonlim2 = [45 -45], there are two intervals of overlap: [-90 -45] and [45 90]. These limits are returned in lonlim in separate rows, forming a 2-by-2 array. In our example (assuming that the latitude limits overlap), lonlim would equal [-90 -45; 45 90]. It still has the form [western-limit eastern-limit], but western-limit and eastern-limit are 2-by-1 rather than scalar. The two output quadrangles have the same latitude limits, but these are replicated so that latlim is also 2-by-2.

To continue the example, if latlim1 = [0 30] and latlim2 = [20 50], latlim equals [20 30; 20 30]. The form is still [southern-limit northern-limit], but in this case southern-limit and northern-limit are 2-by-1.

Examples

Example 1

[latlim, lonlim] = intersectgeoquad( ...
[-40 -60], [-180 180], [40 60], [-180 180])
latlim =
[]

lonlim =
[]

Example 2

Intersection is a single quadrangle:

[latlim, lonlim] = intersectgeoquad( ...
[-40 60], [-120 45], [-60 40], [160 -75])

latlim =
-40    40

lonlim =
-120   -75

Example 3

Intersection is a pair of quadrangles:

[latlim, lonlim] = intersectgeoquad( ...
[-30 90],[-10 -170],[-90 30],[170 10])

latlim =
-30    30
-30    30

lonlim =
-10    10
170  -170

Example 4

Inputs and output fully encircle the planet:

[latlim, lonlim] = intersectgeoquad( ...
[-30 90],[-180 180],[-90 30],[0 360])

latlim =
-30    30

lonlim =
-180   180

Example 5

Find and map the intersection of the bounding boxes of adjoining U.S. states:

usamap({'Minnesota','Wisconsin'})
{@(name) any(strcmp(name,{'Minnesota','Wisconsin'})), 'Name'});
geoshow(S, 'FaceColor', 'y')
textm([S.LabelLat], [S.LabelLon], {S.Name},...
'HorizontalAlignment', 'center')
latlimMN = S(1).BoundingBox(:,2)'

latlimMN =
43.4995   49.3844

lonlimMN = S(1).BoundingBox(:,1)'

lonlimMN =
-97.2385  -89.5612

latlimWI = S(2).BoundingBox(:,2)'

latlimWI =
42.4918   47.0773

lonlimWI = S(2).BoundingBox(:,1)'

lonlimWI =
-92.8892  -86.8059

[latlim lonlim] = ...
intersectgeoquad(latlimMN, lonlimMN, latlimWI, lonlimWI)

latlim =
43.4995   47.0773

lonlim =
-92.8892  -89.5612

geoshow(latlim([1 2 2 1 1]), lonlim([1 1 2 2 1]), ...
'DisplayType','polygon','FaceColor','m') Tips

latlim1 and latlim2 should normally be given in order of increasing numerical value. No error will result if, for example, latlim1(2) < latlim1(1), but the outputs will both be empty arrays.

No such restriction applies to lonlim1 and lonlim2. The first element is always interpreted as the western limit, even if it exceeds the second element (the eastern limit). Furthermore, intersectgeoquad correctly handles whatever longitude-wrapping convention may have been applied to lonlim1 and lonlim2.

In terms of output, intersectgeoquad wraps lonlim such that all elements fall in the closed interval [-180 180]. This means that if (one of) the output quadrangle(s) crosses the 180° meridian, its western limit exceeds its eastern limit. The result would be such that

lonlim(2) < lonlim(1)
if the intersection comprises a single quadrangle or
lonlim(k,2) < lonlim(k,1)
where k equals 1 or 2 if the intersection comprises a pair of quadrangles.

If abs(diff(lonlim1)) or abs(diff(lonlim2)) equals 360, its quadrangle is interpreted as a latitudinal zone that fully encircles the planet, bounded only by one parallel on the south and another parallel on the north. If two such quadrangles intersect, lonlim is set to [-180 180].

If you want to display geographic quadrangles generated by this function or any other which are more than one or two degrees in extent, they may not follow curved meridians and parallels very well. The degree of departure depends on the extent of the quadrangle, the map projection, and the map scale. In such cases, you can interpolate intermediate vertices along quadrangle edges with the outlinegeoquad function. 