GIS Basics
Content:
- Point Data/Localities Lat/Long, converting between formats, & UTMs
- Projections & Datums
- GPS units
- Raster Resolutions
- Data Format Conversions
Point Data
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:
| Format for | latitude | longitude |
| DMS | 37° 52′ 18.01″ N | 122° 15′ 46.43″ W |
| DM | 37° 52.302 N | 122° 15.774 W |
| DD | 37.871669 | -122.262897 |
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.
decimal degrees = degree + (minutes/60) + (seconds/3600)
A word about precision: with the conversion to decimal degrees, implicit coordinate precision is lost and is usually substituted by decimal places to indicate precision.
(table?)
UTM (Universal Transverse Mercator)
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)
World UTM Zones (above)
US UTM Zones (left)
(click for larger alternative image)
If you work in UTMs, you must know your zone!
Map Projections
‘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.
- California– commonly uses Teale Albers Equal-Area (Clarke 1866 spheroid), UTM Zones 10N and 11N, Lambert’s Equal-Area (less frequent)
- North America– Lambert’s Conformal Conic, Albers Equal-Area Conic
- World– Miller Cylindrical, Robinson (National Geographic maps are often in this), Orthographic, Mercator
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).
A few links of interest (many more out there!):
- USGS Guide to map projections, including a pdf version to download
- A nice, practical introduction to Map Projections, with lots of examples and some excellent illustrations of interrupted globes and origami!
- A great introduction to Projections with examples at the Geographer’s Craft site.
- How to identify an unknown coordinate system in ArcGIS 9.2 tutorial from ESRI.
- A fun java applet that changes the projection of a world map with a click of the mouse– interactive learning
Coordinate Conversion between datums
For more information and conversion between NAD 27 and NAD 83, use North American Conversion (NADCON) Utility site.
Coordinate Conversion between projections
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.
GPS data
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.
Knowing your GPS datum is essential; if in doubt, set to the default of WGS84.
Raster Data
Rasters are gridded data and a powerful means of representing spatial data.
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:
1/9 arc-second = 0.00003 degrees = ca. 3 m/pixel
1/3 arc-second = 0.00009 degrees = ca. 10 m/pixel
1 arc-second = 0.00028 degrees = ca. 30 m/pixel
3 arc-seconds = 0.00083 degrees = ca. 90 m/pixel
30 arc-seconds = 0.0083 degrees = ca. 1 km/pixel
Therefore, in a 30 arc second DEM, there is an elevation value for every square kilometer.
Converting ESRI files
- ArcInfo “Interchange” files (*.e00) was an ESRI proprietary format for sharing data (kinda old school now). But there are still occasions when you download data as *.e00, which means it must be converted first to ArcInfo coverage file or shapefile. A standalone app, Import71 will suffice or find the ArcTool equivalent in ArcGIS 9.x or higher.
How-to tutorial posted on Geocommunity website
Download free “Import 71″ utility from ESRI
- ASCII grid to ESRI grid and vice versa
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.
