For point data to be entered into a GIS program such as ArcGIS, coordinates of latitude and longitude need to be in decimal degrees (also abbreviated as DD) instead of, say, degrees minutes seconds (DMS) or decimal minutes (DM). Example of formats: South Latitude (south of equator) and West Longitude (west of prime meridian) values are indicated with negative values in DD. If you have a spreadsheet of degrees, minutes and seconds separated out in their respective columns, then a simple formula in a separate column can convert to decimal degrees. A logical expression for N/S or E/W can be applied to assign the negative values. UTM is another coordinate system based on offset meters from the equator and east of a defined longitude in a Zone system. Converting from UTM to DD, and vice versa, is more complex, several online calculators are available for such conversions as well as macros for excel spreadsheets or standalone utilities. (e.g. UTM Utilities from NGS) ‘Map Projection‘ refers to the various methods used to represent a 3-dimensional world into the 2-dimensions of paper or screen. Datum, a separate calculation and concept, defines the mathematical coordinate system used to represent the globe, and thus the coordinates of latitude and longitude for any given map. All projection methods introduce distortions, however, aspects of area, distance or shape can be minimized or preserved, depending on the purposes of the map. Hundreds of different projections and datums have been defined but for certain regions and uses, there are a handful of commonly used ones to be aware of. Knowing the projection and datum are essential for transforming your data from one to another and overlaying data from different sources. Note: ArcMAP does re-project GIS layers on-the-fly for data visualization but can only do that if the projection is defined for it (Use Project Define in ArcTools). To actually, re-project the data to a new projection, you need to use the Project macro in the Data Management Toolbox (there is a separate one for vector and raster data). For more information and conversion between NAD 27 and NAD 83, use North American Conversion (NADCON) Utility site. TatukGIS offers a free Coordinate Calculator to convert lat/long pairs between geographic coordinate systems, including all the US State Planes. Handy, if you don’t have ArcGIS 9.x. Most GPS units come with an interface application to install and download waypoints, tracks, and routes, so check the manual. A great extension for ArcView 3.x or 9.x is Minnesota’s Department of Natural Resources GPS Application. An easy to use interfacing easing the data flow between a Garmin GPS unit and ArcView, including real-time tracking and other functions. ArcGIS 9.x has built-in GPS compatibility, requiring only a GPS with a serial or usb connector. See online help for details. If you want to directly convert your GPS data into KML, the fastest tool I know of is an online converter, GPS Visualizer, which will take any text file of coordinates and convert to a KML for Google Earth or Google Maps. Digital Elevation Model data now are available from various sources and vastly superior to DEM data from only a decade ago. Usually at resolutions expressed in arc-seconds or the number of seconds per latitude or longitude, thus varying for different longitudes, in general: The ArcGIS Wizard to step through ascii to raster conversion is in the Conversion Toolbox >,To Raster, likewise, to convert from raster data to ascii format wizard is under the From Raster tools. Source.