improved the explanation for CRS

materials/sections/geospatial-vector-analysis.qmd

@@ -124,7 +124,13 @@ glimpse(ak_regions)
 
 [![Source: ESRI](images/projections-esri-blog.jpg){width="90%" fig-align="center"}](https://www.esri.com/arcgis-blog/products/arcgis-pro/mapping/gcs_vs_pcs/)
 
-Every `sf` object needs a coordinate reference system (or `crs`) defined in order to work with it correctly. A coordinate reference system contains both a datum and a projection. The datum is how you georeference your points (in 3 dimensions!) onto a spheroid. The projection is how these points are mathematically transformed to represent the georeferenced point on a flat piece of paper. All coordinate reference systems require a datum. However, some coordinate reference systems are "unprojected" (also called geographic coordinate systems). Coordinates in latitude/longitude use a geographic (unprojected) coordinate system. One of the most commonly used geographic coordinate systems is WGS 1984.
+Every `sf` object needs a coordinate reference system (or `crs`) defined in order to work with it correctly. A coordinate reference system contains both a datum and a projection. 
+
+The datum is how you georeference your points (in 3 dimensions!) onto a spheroid, or the Earth. The Earth is not a perfect sphere and there are many ways to describe its shape. For example, is the Earth shaped like a lemon, lime, or orange? The shape, or datum, that you choose will depend on the scope of your project (for instance, local vs. global) and the specific locations. 
+
+The projection is how these points are mathematically transformed to represent the georeferenced point on a flat piece of paper. Since you will visualize a 3D object onto a 2D space, there will be some distortions depending on the projection that you choose. Analogously, how do you peel the fruit (representing the Earth) and flatten the peel?
+
+All coordinate reference systems require a datum. However, some coordinate reference systems are "unprojected" (also called geographic coordinate systems). Coordinates in latitude/longitude use a geographic (unprojected) coordinate system. One of the most commonly used geographic coordinate systems is WGS 1984.
 
 ESRI has a [blog post](https://www.esri.com/arcgis-blog/products/arcgis-pro/mapping/gcs_vs_pcs/) that explains these concepts in more detail with very helpful diagrams and examples.