Add docstrings

src/geokeys.jl

@@ -50,11 +50,15 @@
   ProjScaleAtCenterGeoKey = 3093
 end
 
-# Corresponding names in the GeoTIFF specification:
-# id - KeyID
-# tag - TIFFTagLocation
-# count - Count
-# value - ValueOffset
+"""
+    GeoTIFF.GeoKey(id, tag, count, value)
+
+GeoKey Entry with KeyID `id`, TIFFTagLocation `tag`, Count `count`
+and ValueOffset `value`.
+
+See [Requirements Class GeoKeyDirectoryTag](https://docs.ogc.org/is/19-008r4/19-008r4.html#_requirements_class_geokeydirectorytag)
+section of the GeoTIFF Spec for a explanation of each field.
+"""
 struct GeoKey
   id::GeoKeyID
   tag::UInt16
@@ -63,14 +67,56 @@ struct GeoKey
 end
 
 # generic values
+
+"""
+    GeoTIFF.Undefined
+
+GeoKey value that indicate intentionally omitted parameters.
+"""
 const Undefined = UInt16(0)
+
+"""
+    GeoTIFF.UserDefined
+
+GeoKey value that indicate user defined parameters.
+"""
 const UserDefined = UInt16(32767)
 
 # GTRasterTypeGeoKey values
+
+"""
+    GeoTIFF.PixelIsArea
+
+PixelIsArea raster type, i.e each pixel of the image is a grid element.
+"""
 const PixelIsArea = UInt16(1)
+
+"""
+    GeoTIFF.PixelIsPoint
+
+PixelIsPoint raster type, i.e each pixel of the image is a grid vertex.
+"""
 const PixelIsPoint = UInt16(2)
 
 # GTModelTypeGeoKey values
+
+"""
+    GeoTIFF.Projected2D
+
+Projected CRS model type.
+"""
 const Projected2D = UInt16(1)
+
+"""
+    GeoTIFF.Geographic2D
+
+Geographic 2D CRS model type.
+"""
 const Geographic2D = UInt16(2)
+
+"""
+    GeoTIFF.Geocentric3D
+
+Geocentric 3D CRS model type.
+"""
 const Geocentric3D = UInt16(3)