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====== Map Projection ======

Changing the **Map Projection** allows you to control the way in which your map data is displayed on your screen.  Projection is a method of representing a spherical body (such as the Earth) on a two-dimensional surface (such as a computer screen).  There are many different projections available, each of which represents the spherical curves of Earth in different ways.

Because every projection distorts the data in some areas to a certain degree, choosing the right projection for the maps you want to view is important.  For example, if you will be creating maps of a very specific local region of the United States, you might want to use a State Plane projection which focuses on that region.  This will minimize distortion of the region you're working in at the expense of distorting other areas of the world.  Because you will not need to display those other areas of the world, the distortion there is acceptable.

The GIS Editor supports just about any projection imaginable, and makes the most common projections easy to access via the map's **Projection Configuration** dialog, which is described below.  For more information about the concept of projecting map data, we recommend [[http://en.wikipedia.org/wiki/Map_projection|this Wikipedia article]].

===== Video Overview =====
{{youtube>zKXfr1kcr20?720|Configuring the projection of your map.}}

===== Setting the Map Projection =====
In the GIS Editor, your map will //reproject// all of your layer data if necessary, no matter what projection it is in, to the projection you specify.  This way, you can load layers whose data are formatted in a variety of different coordinate systems and project them all on-the-fly into a single unified projection of your choosing.

By default, new maps in the GIS Editor use the Geographic (Latitude/Longitude) projection, also known as WGS84 or EPSG:4326.  To view or change the current projection of your map, click the Map Projection button on the Home tab of the GIS Editor's [[Using the Ribbon Bar|ribbon bar]].  This will bring up the Projection Configuration dialog.

==== Projection Configuration ====
The Projection Configuration dialog consists of two tabs:

  - Common Projections
  - Other Projections

If you want to change the projection of your map, you will use either the Common Projections tab //or// the Other Projections tab to set the projection you want.  The most common projections that users will want to access most often can be easily chosen from the Common Projections tab.  If you are using a very specific local projection, an obscure projection or a custom projection for which you have a .prj file or a Proj4 string, you will want to use the Other Projections tab.

=== Common Projections ===
The GIS Editor provides easy access to a variety of common projections through this tab.  Those projections include:

  * **Geographic (Latitude/Longitude)** - also known as WGS84 or EPSG:4326, this projection is commonly used by GPS satellite navigation and by NATO for geodetic surveying. It is the default projection for new maps in the GIS Editor.
  * **Google Maps / Bing Maps / OpenStreetMap** - also known as Spherical Mercator or Web Mercator, this is the projection used by most online web map services.  You will most likely need to use this projection if you want to overlay your layer data on top of a base map from one of the aforementioned providers.
  * **State Plane** - a set of 124 geographic zones designed for specific local regions of the United States. [[http://en.wikipedia.org/wiki/State_Plane_Coordinate_System|More information here.]]
  * **UTM** - a set of 60 geographic zones covering the entire Earth, each of which consists of a six-degree band of longitude. [[http://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system|More information here.]]

These projections are made available via a series of easy-to-use dropdown menus.  Some projections require you to set various options including Datum, Zone and Unit, while others (such as Geographic and Google Maps) do not require any further options to be set.

As you make your selections, the GIS Editor will automatically generate the Proj4 projection string that corresponds to the common projection that you have selected.  This string is displayed in a read-only text area beneath the dropdown menus.  You will most likely not need to refer to the Proj4 information; though it is particularly useful for software developers who are programming against ThinkGeo's [[http://gis.thinkgeo.com|Map Suite GIS components]].

Click the OK button at the bottom of the dialog to apply the projection you have chosen.

=== Other Projections ===
If you need to use a projection that is not available on the Common Projections tab, switching to the Other Projections tab will open up a wider range of options for you.

  * **EPSG Search** - allows you to find a projection according to its EPSG number or description.  This list has some partial overlap with the ESRI search.
  * **ESRI Search** - allows you to find a projection according to its ESRI number or description.  This list has some partial overlap with the EPSG search.
  * **Custom** - allows you to load a custom .prj file which describes the projection for your data.  For more information about .prj files, see [[http://en.wikipedia.org/wiki/Shapefile#Shapefile_projection_format_.28.prj.29|this Wikipedia article]].  Alternatively, when the Custom option is selected, you can also paste the Proj4 projection string for your intended projection into the Projection String text box at the bottom of the dialog.

Click the OK button at the bottom of the dialog to apply the projection you have chosen.

===== Setting the Internal Projection of a Layer =====
Each layer you load onto your map has an //internal projection// which is separate from your map's projection.  The internal projection identifies the coordinate system in which that layer's data has been stored.  The internal projection must be set properly, otherwise the layer data will display in the wrong place on your map (or may not appear at all).

Loading a layer with an improperly-set or unknown internal projection is uncommon, but does happen.  This typically only occurs if you have layer data that has been stored in an obscure coordinate system, or if the layer's projection was set incorrectly by mistake.

If you know the true projection of the layer, you can correct this problem by right-clicking on the layer in the [[Using the Layer List|Layer List]] and choosing Layer Projection.  This will open the same Projection Configuration dialog we have discussed above, with the difference being that this time you are setting the internal projection in which that specific layer's data is stored.

**Please use caution when setting a layer's internal projection**, as setting this value incorrectly can cause the layer to display in the wrong place on your map or not at all.

===== Further Resources =====
If you are unsure what projection you should use, [[http://spatialreference.org/|spatialreference.org]] is a good resource for finding spatial references in a variety of formats.
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