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I need help!
- Open help hours in the GIS lab (Rotch Library, 3rd floor): Monday-Thursday: 1-5pm, Friday: 2-4pm
Map projections & Coordinate Systems
Map projections and coordinate systems tell GIS software where to place data on a map.
- For a shapefile, projection information is stored in the file that ends in .prj.
- For an image (.jpg, etc.) that has been georeferenced, projection information is stored in the .jgw and .xml files. (Only ArcMap can read this type of projection information.)
- For other raster data, projection information is saved in a .tfw and .xml file.
There are two types of coordinate systems:
- Geographic: Use a 3D, spherical surface to define locations and units are in degrees.
- Projected: Use a flat, 2D surface and units are in meters, miles, etc. Area, length, direction, or angles will be distorted. When transforming data from geographic to projected, choose a map projection centered where your data are located to minimize distortion.
Learn about the different map projections and how they distort data.
Make a color-coded map
Make a color-coded map based on any of the columns in your attribute table.
To view the attribute table: Right click > Open Attribute Table
Create a new vector layer
In ArcMap, when you begin editing, if you receive a warning message about the coordinate system of the data layer not matching the data frame, you can change the coordinate system of the data frame:
- Stop edting and click View > Data Frame Properties
- Select the Coordinate System tab
- Click the Globe icon in the upper right and Import...
- Select the shapefile you just created
- Click Ok.
The coordinate system of the data frame does not change the coordinate systems of the individual data layers. Rather, it changes how the data are displayed on the screen. You may see your data stretch and distort as you change the coordinate system of the data frame.
Find Data & Maps
Tips for finding data on the web:
- Use “GIS”, “data”, or “map” as some of your search terms. Example: wind data gis, US census gis
- If you are looking for data from a specific location, search for a town/county/state/country GIS organization and contact them.
- Many cities have open data portals that contain GIS and other data. Example: Boston open data
- When searching for foreign data, search in the language native to that country.
- Search for data for different administrative "levels". Example: Spain vs. Barcelona
- Look for universities near your area of interest and contact researchers. They may be willing to share their data!
- Vector data is most commonly in the format of a shapefile.
- Although called "a" shapefile, it consists of many different files with the same names, but different file extensions.
- GIS software will access all these files in order to display your data.
- Keep all files together if you move them.
- Unzip the files before opening them in GIS software.
When you georeference raster data, you define its location using map coordinates.
- Do this manually by picking a distinct point on your image and indicating where this point aligns with a data layer on your map.
- You can georeference to a basemap, another image, raster, or vector data.
- Georeferencing raster data allows it to be viewed with other geographic data from the same location.
- Once you click "Update Georeferencing" the map projection information is permanently stored in .jgw and .xml files. The image will open in the proper location in a new ArcMap document, however other programs (such as QGIS) may not be able to read these projection files.
Export data to other formats