Map projections
Basic terms
A Geographic coordinate system (GCS) is a reference framework that defines the locations of features on a model of the earth. It’s shaped like a globe—spherical. Its units are angular, usually degrees.
GCS defines where the data is located on the model of Earth’s surface.
A Projected coordinate system (PCS) is flat. It contains a GCS, but it converts that GCS into a flat surface, using math (the projection algorithm) and other parameters. Its units are linear, most commonly in meters.
PCS tells the data how to draw on a flat surface, like on a paper map or a computer screen.
A map projection is the mathematical algorithm that defines how to present the round earth on a flat map. It is one parameter in a projected coordinate system (PCS).
Projection on the fly
Projection on the fly is what ArcGIS does to resolve conflicts when your data is in a different coordinate system than your map. It does this so the data can draw in the correct place on the map. Projection on the fly ensures that the data draws in the map’s coordinate system, even though it is still stored in other coordinate systems. ArcGIS will always apply projection on the fly when it’s needed. It can’t draw the data on your map otherwise.
WKID
The Well-Known ID (WKID) is a unique number assigned to a coordinate system. You can find the WKID in the Coordinate Systems Details window. Once you know this number, it’s a handy way to search for the coordinate system later. The authority of the WKID will either be EPSG or Esri but these numbers don’t overlap, so there’s no need to worry about which authority defined the ID. Most geographic information systems (GIS) and GIS libraries use EPSG codes as Spatial Reference System Identifiers (SRIDs) and EPSG definition data for identifying coordinate reference systems, projections, and performing transformations between these systems.
WKT
The Well-Known Text (WKT) is a text markup language that defines all necessary parameters of a coordinate system. To eplore the WKT of the coordinate system, save its projection file (.prj) and open it in a text editor.
Sources:
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Web Mercator distortion
The Web Mercator projection, an adapted version of the Mercator projection, has become the default map projection for web mapping. Unlike its predecessor, it employs a spherical formula consistently across all scales. Major online map providers, such as Google Maps, CARTO, Mapbox, OpenStreetMap, Esri, and others, widely employ this projection.
The primary advantage of the Mercator projection, and consequently the Web Mercator, is its preservation of direction, providing users with the valuable knowledge that north is consistently oriented upwards. Despite even distortion throughout most areas, as one moves away from the equator, distortion intensifies, causing significant stretching toward the poles. Consequently, the Web Mercator projection is unsuitable for polar displays. Due to these apparent distortions, it is not recommended for spatial analysis or area calculations.
Assignment 01
Shapes of Iceland
TASK:
Make a map showing distortions and area of Iceland in following projections:
- Bonne
- Winkel–Tripel
- Mollweide
- Cube
- Spilhaus
- Robinson
- Fuller
- Plate Carée
- Mercator
For comparison, add a map of Iceland in the map projection that is recommended for displaying Iceland.
Your map should look similar to this map.
In technical report answer following questions:
- Which projection distorts the shape of Iceland the most/the less? Compare the total area of Iceland in these projections with its actual size.
- Name at least three map projections, that are most suitable (according to their accuracy) for displaying Iceland. Where can you find the information about these map projections?
How-To
To calculate the total area of Iceland in each projection you have to do following steps:
- Use the Project tool to project spatial data to PCS.
- In the attribute table, add a new field for each projection (e.g. area_merc, area_bonne, area_cube, etc.).
- Use the Calculate Geometry tool to calculate the area of Iceland in all desired projections.
DATA SOURCES:
SUBMISSION FORM:
- technical report + 1 map in PDF format (submit by 16/03, send to petra.justova@fsv.cvut.cz)
INSTRUCTIONS:
- Set the projection of Map (Properties-Coordinate Systems) and add states_provinces layer.
- Use Definition Query to filter the features to work only with your area of interest.
- Use optional symbolization of the layer.
- Duplicate the original map and create 9 more maps and change the projection in each map to a different one.
- Add all map frames to layout and make sure each has the same scale (1:10,000,000).
- In New Layout (A3 Portrait) insert the Map Title, Scale and Credits
- Label each shape with the name of the map projection used.
- Export Layout in PDF Format