diff --git a/README.md b/README.md
index c6474d9..240b482 100644
--- a/README.md
+++ b/README.md
@@ -6,7 +6,7 @@ GIS tools for Swift, including a [GeoJSON][3] implementation and many algorithms
 
 ## Notes
 
-This package makes some assumptions about what is equal, i.e. coordinates that are inside of `1e-10` degrees are regarded as equal. See [GISTool.equalityDelta][5].
+This package makes some assumptions about what is equal, i.e. coordinates that are inside of `1e-10` degrees are regarded as equal (that's μm precision and is probably overkill). See [GISTool.equalityDelta][5].
 
 ## Requirements
 
diff --git a/Sources/GISTools/GISTool.swift b/Sources/GISTools/GISTool.swift
index 08b46b3..741e03d 100644
--- a/Sources/GISTools/GISTool.swift
+++ b/Sources/GISTools/GISTool.swift
@@ -17,7 +17,7 @@ public enum GISTool {
     /// Mercator projection origin shift.
     public static let originShift = 2.0 * Double.pi * GISTool.equatorialRadius / 2.0 // 20037508.342789244
 
-    /// The accuracy for testing what is equal.
+    /// The accuracy for testing what is equal (μm precision, mainly to counter small rounding errors).
     public static let equalityDelta: Double = 1e-10
 
     /// The length in pixels of a map tile.