Merge pull request #1067 from smkerr/patch-1

Update 02-spatial-data.Rmd

02-spatial-data.Rmd

@@ -1069,7 +1069,7 @@ This has important consequences, as demonstrated in Section \@ref(reproj-geo-dat
 
 The surface of the Earth in geographic coordinate reference systems is represented by a spherical or ellipsoidal surface.
 Spherical models assume that the Earth is a perfect sphere of a given radius -- they have the advantage of simplicity but, at the same time, they are inaccurate as the Earth is not exactly a sphere.
-Ellipsoidal models are slighly more accurate, and are defined by two parameters: the equatorial radius and the polar radius.
+Ellipsoidal models are slightly more accurate, and are defined by two parameters: the equatorial radius and the polar radius.
 These are suitable because the Earth is compressed: the equatorial radius is around 11.5 km longer than the polar radius [@maling_coordinate_1992].^[
 The degree of compression is often referred to as *flattening*, defined in terms of the equatorial radius ($a$) and polar radius ($b$) as follows: $f = (a - b) / a$. The terms *ellipticity* and *compression* can also be used.
 Because $f$ is a rather small value, digital ellipsoid models use the 'inverse flattening' ($rf = 1/f$) to define the Earth's compression.