Mapping GIS Projections Digital Scholarship Lab Florida Institute of Technology September 2017

How do we go from this to this? https://www.flickr.com/photos/gsfc/6760135001/in/photostream/ www.maps.google.com

Today’s Agenda Basic Concepts & terminology Spheroids, Datums, Graticules, Geographic Coordinate Systems, Projections, Spatial Reference Systems, etc. The challenges of projections Working with unknown or inconsistent projections Plotting point data

A common misconception The earth is a sphere. Actually, the earth has an “equatorial bulge”. Radius at equator: 6378 kilometers Radius at the poles: 6357 kilometers The earth is better approximated as an oblate spheroid. Radii from NASA Earth Fact Sheet (https://nssdc.gsfc.nasa.gov/planetary/factsheet/earthfact.html)

Datums A datum is a 3-dimensional mathematical representation of the earth, including: A spheroid approximating the earth’s shape The spheroid’s position relative to the center of the earth. Examples of common datums: WGS 84 (World Geodetic System, first established in 1984) NAD 83 (North American Datum of 1983)

Geographic Coordinate Systems Datum + Graticule  Geographic Coordinate System The graticule is a reference system used to define locations on the earth’s surface. Consists of a gridwork of lines of longitude and latitude. Locations are defined in degrees e.g. location of the Evans Library: 28.066° N 80.623° W Angles used to define the lines are measured relative to the datum’s center.

Geographic Coordinate Systems: Pros and Cons Benefits of Geographic Coordinate Systems Easily stored and accessed datasets Good for rapidly generating maps Limitations of Geographic Coordinate Systems Not good for accurately reproducing the shapes and areas of map features, or distances and directions between them.

Projected Coordinate Systems Derived from Geographic Coordinate Systems Represents the 3-dimensional earth on a flat (2-dimensional) surface. Locations of features expressed in vertical and horizontal distances (miles, meters, kilometers) from some reference point. For world maps, the reference point is typically the intersection of the Equator and the Greenwich Meridian. Allows distances and areas to be accurately measured. Therefore supports spatial analyses requiring the calculation of directions, distances and areas.

Projected Coordinate Systems: Challenges Accurately representing a 3-dimensional earth on a flat (2-dimensional) surface is not easy. All projected maps require tradeoffs in between the accuracy of: Feature shapes Distances Directions Areas The choice of what type of projection to use will often depend upon what you are trying to analyze. “Why all world maps are wrong”

Projected Coordinate Systems in ArcGIS: Why do they matter? Type of projection should make sense in terms of your map’s purpose. What’s important for your map? Accurately showing a specific part of the world? Calculating distances and/or areas? etc. Data cannot be displayed on a map without a spatial reference system. If the wrong spatial reference system is specified, the data will be displayed incorrectly on the map. Analyses performed using multiple sets of data with differing spatial reference systems will tend to have inaccurate results.

What does ArcGIS mean by a “spatial reference system”? A spatial reference system can be either: A Geographic Coordinate System or A Projected Coordinate System

Geographic & Projected Coordinate Systems in ArcGIS A new ArcMap data frame initially has no assigned spatial reference system. Right-click on data frame name. Select “Properties…” “Coordinate System” tab No coordinate system

ArcMap Options for Geographic & Projected Coordinate Systems ArcMap offers many options for: Geographic Coordinate Systems Projected Coordinate Systems Geographic Coordinate Systems are typically in units of degrees Projected Coordinate Systems are most commonly in units of meters or feet.

Assigning a Spatial Reference System Two options for establishing a spatial reference system. Directly assigning to the data frame (see previous slides). Importing an existing data set into the data frame Data frame will use the spatial reference system of its first layer/data set. Note: Spatial reference system for 1st data layer is also used for the data frame.

Data with Missing Spatial Reference System What happens if you import data that doesn’t indicate a spatial reference system? You get a warning message ArcMap draws the shapes, but ArcMap does not know: Where the shapes are located The units in which they are expressed Their areas How far apart they are.

Defining a Layer’s Projection If you can determine the correct projection for a data layer, you can assign the projection to the layer, as follows: Click on the ArcToolbox icon “Drill down” to the “Define Projection” tool “Data Management Tools”  “Projections and Transformations”  “Define Projection” Double-click on “Define Projection” Fill in the “Define Projection” form, click on “OK”

What if the wrong projection is associated with a data set? If a data set incorrectly indicates that it was created using a certain projection, it will display incorrectly on the map.

What happens when different map layers have different projections? When adding a new layer of data to a map, if it has a different projection than existing layers, You will get a warning message If you continue, ArcMap will re-project the new layer “on the fly”. The image of the new layer on the map will be modified so that it aligns with previous layers. However, the actual data for the different layers will still be based upon different types of projections. When you start performing spatial analyses (involving distances, areas, etc.), this can result in inaccurate results.

How can you resolve conflicting projections? Click on the ArcToolbox icon “Drill down” to the “Project” tool “Data Management Tools”  “Projections and Transformations”  “Project” Double-click on “Project” Fill in the “Project” form, click on “OK” Use your new layer (with the changed projection) during your spatial analyses

Plotting Field Data To display data points expressed in degrees of longitude & latitude: Coordinates should be in decimal degrees (not degrees, minutes & seconds) Degrees of longitude should be negative in the Western Hemisphere and positive in the Eastern Hemisphere. Degrees of latitude should be negative in the Southern Hemisphere and positive in the Northern Hemisphere. The data frame’s spatial reference system should be the geographic coordinate system used when recording the longitude and latitude values No projected coordinate system should be specified. Once the points are displayed on the map, a projected coordinate system may be introduced.

Want to learn some more? ESRI Projection Basics for GIS Professionals: http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/ Projection_basics_for_GIS_professionals/003r00000002000000/ USGS (U.S. Geodetic Survey) explanation of Map Projections: https://egsc.usgs.gov/isb//pubs/MapProjections/projections.pdf USGS Universal Transverse Mercator (UTM) Grid fact sheet: https://pubs.usgs.gov/fs/2001/0077/report.pdf

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