IMHO you shouldn't use System.Spatial
. It's an half-baked library created for OData interoperability with Spatial, namely for WCF Data-Services.
You can check here for more information. Also, most of the classes on that library are abstract, hence up-to-you to be implement most-of-it.
Also, and even worse, they're incompatible with Entity Framework spatial support.
I'm not sure what kind of project you're on but I would suggest these guidelines:
if you just need to calculate distances between points just copy an implementation of the Haversine Formula or Vincenty's Formula in C#.
if you need spatial on DB just go with Entity Framework 5/6.
if you need further spatial operations in c# use something like NetTopologySuite. There's a NuGet for it and it's really simple to use.