If you have data that is already in a projected coordinate system (something other than WGS84, e.g. UTM or State Plane) than you can still follow the basic Add XY Data instructions. By default, GPS receivers record data in latitude-longitude using the WGS84 (World Geodetic System 1984) datum (WGS84 is “an earth fixed global reference frame, including an earth model.” See ). Many GPS receivers allow the user to view their location in local coordinates as well (e.g., UTM, State Plane, etc.), performing a translation from WGS84 to the desired coordinate system (and datum). Similarly, some GPS software packages allow you to convert to the coordinate system of your choice when you export your data. Often, however, GPS data can only be exported in WGS84, typically as a text file with X-Y coordinates along with an ID, a time stamp, altitude and/or other information. Steps for bringing this “raw” GPS data successfully into ArcGIS are outlined below. Note that this information is somewhat written for our local datasets (i.e., converting WGS84 GPS data to the commonly used coordinate systems in the WWU area: UTM zone 10 North, and State Plane WA North, NAD83 or NAD27). The process of getting your data out of a GPS receiver will vary depending upon the type of receiver and/or the GPS software being used. For the purposes of this tutorial, it is assumed that the GPS data is in a standard ASCII text file, with commas or tabs delimiting the columns of information (not spaces). These columns should include (at the very least) an X (Longitude) and Y (Latitude) field in decimal degrees (if your data is in degrees, minutes and seconds you will need to convert it to Frequently, GPS data will need to be used in conjunction with local data sets using a NAD83 or NAD27 datum. (NOTE: While the instructions below use ArcMap, ArcToolbox can also be used for defining the projection of your data and/or for re-projection from one datum to another, however, make sure to use the proper In brief, there are two methods of getting WGS84 GPS data into NAD83 or NAD27 coordinate systems The first of these is to assign a NAD83 datum for the GPS data (even though the data is WGS84) (See discussion on why a NAD83 datum works for WGS84 data below) and provides a simpler, more rapid process for viewing and using the GPS data. The second option requires a few additional steps but should produce slightly better results. Experimental tests indicate that the difference between the two methods to be less than 2 meters. Given the inherent inaccuracy of GPS, the first method may well be “good enough” for most applications. These methods are outlined below, with more complete details on the following pages: Complete the Steps detailed in 2a above (bringing WGS84 GPS data into ArcMap and converting it to NAD83 data, exported to a shapefile). For our part of the world, WGS84 and NAD83 are very close to being the same. This is why Method 1 above (simply defining your WGS84 data as NAD83) works. must be applied to the data. In the case of WGS84 to NAD27, this involves a mathematical calculation that inevitably introduces a certain amount of error. On the other hand, for Geographic Transformations between NAD83 and NAD27 a much more precise “grid-based” transformation can be used (NADCON: see also ). Because the shift between the two datums is not uniform across the entire country there is no single equation that can be applied. Instead, the grid-based transformation uses a series of locally appropriate coordinate calculations (based on a data file which contains the actual datum shift at each point in a network of grid points) to more precisely convert data. Since WGS84 and NAD83 are almost identical, calling your GPS data NAD83 allows ArcMap to use the more accurate grid-based method of transformation, and will typically give better results than transforming directly from WGS84 to NAD27. to convert from NAD83 to NAD27. This two-step process takes into account the slight differences between WGS84 and NAD83, while also taking advantage the superior NADCON geographic transformation to convert to NAD27. Source.