[GIS] Projecting data without spatial reference in ArcGIS Desktop

arcgis-desktoparcmapcoordinate system

I have a raster file downloaded from here. When I load it into ArcMap, it says the data is missing spatial reference, and that it can be drawn in ArcMap but not projected. Nevertheless, using the Define Projection tool, I can assign a coordinate system that seems to work pretty well.

How does ArcGIS Desktop do this if there was no spatial reference to begin with?

Is this a valid approach to data missing spatial reference?

Best Answer

How does ArcMap do what? It can display data without a spatial reference / coordinate system (SR / CS), but try to add other data that's really in a different CS. They're not going to overlay unless you happen to set the data frame's coordinate system to the unknown data's CS. ArcMap does not reproject data that has no spatial reference / coordinate system. ArcMap simply displays it.

When you define the coordinate system, you're just updating the metadata. In essence, you're saying, "I know/think/hope that this is the correct coordinate system for the data."

It's often pretty easy to figure out the coordinate system for a dataset if it's projected, although you can still get the geographic coordinate system portion wrong. Get the projected coordinate system wrong, the data can end up thousands of meters or feet offset.

If the data's in latitude/longitude, decimal degrees, it's easier to make a mistake because at most the data may be offset a few hundred meters, and is often much closer. Without excellent reference data to compare against, you're just guessing.

Note: The best solution is to track down the data provider and get them to \c\o\u\g\h \u\p tell you what the coordinate system is.

Related Solutions

[GIS] Setting spatial reference to OSGB_1936_British_National_Grid in ArcGIS Desktop

In ArcGIS 10, if you go into, for example the Data Frame Properties, and go to the "Predefined" co-ordinate systems, you can click on two co-ordinate systems you mentioned in the question - OSGB 1936, and British National Grid, if you click "Modify" on both you will see that they actually both use the OSGB 1936 Datum (click on "Modify" again on BNG to see the Datum).

You probably want to use the British National Grid Co-ordinate system, because it is the basis of all Ordnance Survey Grid references- basically the projection is a set of mathmatical formulae that maps real world positions onto the co-ordinate system that is used in the UK. Not entirely sure what the OSGB_1936 co-ordinate system would be, but the major difference between a geographic and projected co-ordinate system is that:

"Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions" (ArcGIS 10 Help File).

This obviously makes it much more useful for precision applications such as plans, although this accuracy will generally only be high over a relatively small area compared to a geographic co-ordinate system, which is suitable for describing a point with (relative) accuracy over the whole earth's surface.

[GIS] What to set spatial reference to after projecting to GCS_WGS_1984

You need to use Define Projection and search for your original coordinate system. The one you are looking for can be found in:

Select >> Geographic Coordinate Systems >> Europe >> GCS_CH1903

Name: GCS_CH1903
Angular Unit: Degree (0.017453292519943299)
Prime Meridian: Greenwich (0.000000000000000000)
Datum: D_CH1903
Spheroid: Bessel_1841
Semimajor Axis: 6377397.155000000300000000
Semiminor Axis: 6356078.962818188600000000
Inverse Flattening: 299.152812799999990000

After you can use the tool Project tool (Projections and Transformations >> Feature >> Project), to change from the Swiss Geographic (GCS_CH1903) to WGS84

Best Answer

Related Solutions

[GIS] Setting spatial reference to OSGB_1936_British_National_Grid in ArcGIS Desktop

[GIS] What to set spatial reference to after projecting to GCS_WGS_1984

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