 
Background. One of the simplest applications of GIS to plant disease management is the placement of observation or sample points on a base map, linking data to the points, and displaying the results. Therefore, on this page and its links, we provide a step by step procedure on how we do this. Step 0.
An
introduction to GIS, ArcView projects,
Step 1. Creating and displaying base maps.The explanations above are based on ArcView GIS 3.2 for PC's by Environmental Systems Research Institute, Inc. (ESRI). Although ArcView can use a variety of data formats, the native data format for ArcView is called a shapefile. A shapefile really consists of three to five files located in a file folder - all with the same name, but with different extensions. We are interested in spatial patterns across regions that may span several hundred kilometers. Therefore, we usually do not need to locate sample points with a high degree of precision. We use inexpensive handheld GPS units and maps to determine the coordinates for the center of fields that we sample. The accuracy of the handheld GPS is approximately 100 m or less. Most of the fields that we sample are 10 acres (4 hectares) or larger. A square 10 acre field has sides 200 m in length. It is important to be sure the shapefiles, the
coordinates that you create for your points, and all the relevant baseline
data are in the same coordinate system. Much of our baseline data
for Arizona uses the North American Datum of 1927 (NAD27) whereas a lot
of the new baseline data are in the North American Datum of 1983 (NAD83).
There can be a bias of over 100 m depending on the map datum. GPS
units have a setting for map datum which you will want to match with your
baseline data (details on map
datum). ArcView 3.2 has the ability to project from one
coordinate system to another.
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