Spatial data includes information that allows you to position it on earth. This information is part of a coordinate system, which provides a frame of reference for the data to achieve the following:
- Determine each feature's location on the earth.
- Align the data relative to other data for spatially accurate display and analysis.
- Add to and edit the data.
All spatial data in ArcGIS Online is created in a horizontal (x,y) coordinate system. It can optionally have a vertical (z) coordinate system. The units of the coordinates vary by the coordinate system.
For a list of geographic and vertical coordinate systems that ArcGIS software supports, see the Geographic and Vertical Coordinate System Tables PDF.
Horizontal coordinate systems
Horizontal coordinate systems locate data across the surface of the earth. They can be geographic or projected.
Geographic coordinate systems are based on a three-dimensional ellipsoidal or spherical surface, and locations are defined using angular measurements, usually in decimal degrees, measuring degrees of longitude (x-coordinates) and degrees of latitude (y-coordinates). The location of data is expressed as positive or negative numbers: positive x- and y-values for north of the equator and east of the prime meridian and negative values for south of the equator and west of the prime meridian.
Projected coordinate systems are planar systems that use linear measurements for the coordinates rather than angular units. A projected coordinate system is composed of a geographic coordinate system and a map projection and its parameter values. A map projection contains mathematical calculations that convert the angular geodetic coordinates of the geographic coordinate system to Cartesian coordinates of the planar projected coordinate system.
Tip:
You can download the list of projected coordinate systems supported by ArcGIS.
Vertical coordinate systems
Vertical coordinate systems provide a reference for z-coordinates, which are measurements of the height or depth of features. They are always measured in linear units such as meters or feet. Using a vertical coordinate system improves locational accuracy in analysis and editing; however, they are not always supported or included in ArcGIS Online.
Vertical coordinate systems are either gravity-based or ellipsoidal as follows:
- Gravity-based vertical coordinate systems are more commonly used. They often reference a mean sea level calculation (or, in some cases, are derived from the level of a single point).
- Ellipsoidal coordinate systems reference a mathematically derived spheroidal or ellipsoidal surface. Because they are calculated using a mathematical model, ellipsoidal coordinate systems are simpler than gravity-based vertical coordinate systems, but they may lack significant accuracy, especially in large-scale applications. For example, a stream in a large-scale map may appear to flow in a different direction using an ellipsoidal vertical coordinate system.
When you use an ellipsoidal vertical coordinate system, you must ensure that it matches the horizontal geographic coordinate system. For example, if z-value height is defined in NAD 1983, the geographic coordinate system or the geographic coordinate system within a projected coordinate system must also be defined in NAD 1983 and not in WGS 1984.
Vertical coordinate systems in a global scene must be ellipsoidal, with one exception. They can be gravity-based only if they cover a full-world extent. EGM2008 Geoid and EGM96 Geoid are examples of global gravity-based vertical coordinate systems.
Map projections
A map projection is how the data displays on a flat surface, such as a piece of paper or a digital screen. Mathematical calculations in the map projection are used to convert the coordinate system used on the curved surface of earth to one used on a flat surface.
In Map Viewer, the map projection is defined by the projection of the basemap. To change the map's projection, switch the basemap.
How coordinate systems affect use of a layer
When you add layers to Map Viewer or Scene Viewer, ArcGIS Online reprojects the map layers so they display together on the existing basemap. It does not alter the projection of the data; it applies the reprojection in the map or scene. Although this makes it easier to view data together, it can negatively affect drawing performance because Map Viewer or Scene Viewer has to perform calculations each time you pan, zoom, or redraw a layer.
For editable feature layers, the coordinate system can impact editors' ability to accurately place or alter features. If the data in the layer is regional, not global, store it in a coordinate system that is appropriate for the specific region. For example, in the United States, State Plane Coordinate Systems are defined that project data appropriately for the shape and area of a state or predefined zones in a state.
In Scene Viewer, the coordinate system of the basemap affects whether the scene you create is a global scene or a local scene. See Choose a global or local scene for more information.
Identifying an item's coordinate system
The coordinate system defined for the following item types appears on the Overview tab of the item's page:
- ArcGIS Server image services
- Hosted feature layer
- Hosted tile layer
- Map image layer
- Web map
Having this information helps you determine how best to use it, and if you should (when available) export the data, transform it, and republish.
This information will be available for additional item types in future releases.
Tip:
For layer item types that do not currently have this information on the item page, you can look up the well-known ID (WKID) or well-known text (WKT) of a service's coordinate system by clicking the View button next to the URL of the item in the Details section of the Overview tab of the item page. You can use the WKID or WKT to look up the coordinate system in a web search engine.
For example, if the Spatial Reference value on the ArcGIS REST Services Directory page is 102100 (3857), the coordinate system WKID is 3857.