If you take only one point away from this exercise, it should be that there are a multitude of ways to reference points on the surface of the Earth, and that if you combine data sets, either (a) they must all agree in all parameters or (b) you must know exactly what you are doing. For the case of (b), in the absence of having access to a geodesist, we recommend two resources to start with. First are the texts Understanding Map Projections
by Esri and Lining Up Data
by Margaret M. Maher also published by Esri. The second resource is to navigate to the Esri website, www.esri.com
. Then select the support page and type 21327 in the Search Support box. You should see an article entitled “(How To:) Select correct datum transformation when projecting between datums.” Reference material, labeled “Related Information,” will be found on this Web page.
Look at Geographic Data on the web
Starting with some of the Web address in the Overview of this chapter, locate five spatial datasets both in your geographic area and on a topic interesting to you. Make a list of those sources you find.