Hi, please help!
How do you change the projection of a shapefile in ArcGIS 9? Changing the co-ordinate system seems to have no effect on the appearance of the shapefile. I'm using ArcCatalog to change the properties of the file, selecting "XY Coordinate System" and selecting a new predefined coordinate system.
Thanks very much!
Sounds like your doing it the right way. Are you sure you want to change the projection or the datum? What are you changing from and to?
The shapefiles I want to use are supplied by DECC (http://og.decc.gov.uk/en/olgs/cms/data_maps/offshore_maps/offshore_maps.aspx) and have the disclaimer "The shapefiles below are constructed by connecting GCS coordinates with a straight line rather than a geodetic arc (takes into account the curvature of the earth). They are therefore not suitable for defining legal boundaries or measuring areas." so the UK coastline, for example, looks kind of squashed.
EDIT: And if possible I want to be able to measure areas!
The datum is "D_European_1950" and spheroid is "International_1924". I want to use WGS 1984 Mercator (I think!) but changing the coordinate system to this makes no difference to the shapefile's appearance.
Doesnt sound like you really understand what a projection or datum is.
Just changing the properties won't change the spatial relationship of the coordinates in the file. It is what it is.
You may be able to reproject it or Arcgis used to transform on the fly into whatever your working in.
nOOOOOOOOOOOOOOOOOOOOO don't do that. You need to use the Project tool in ArcToolbox, in Data Management Tools>Projections and Transformations>Feature>Project. Use this in ArcMap or ArcCatalog to select your original shapefile and specify the output shapefile and projection. You will quite likely also need to use a Transformation from the transformation drop down menu, to find the right one it is best to get an understanding of the EPSG database, though there are 'standard' ones, for example if you are going between British National Grid and WGS84 the 'Petroleum' one will see you right unless you are requiring mega-preicision.
Without using a transformation you can introduce shifts into your data, for example going between BNG and WGS84 you can get a nice 100m+ shift in data if you don't use a transformation, which can really mess things up later on.
Lack of understanding of coordinate systems and transformations is the biggest problem I come across as a GISser, worth getting right from the start to save you going back later.
Doesnt sound like you really understand what a projection or datum is.
Correct! 
You need to actually warp the shapefile, currently you're keeping the same geometry. http://support.esri.com/en/knowledgebase/techarticles/detail/21447
From memory DECC data will be using ED50 datum, though you can check, to make it look 'proper' and measure areas and distances you need to project the data, so you could try a UTM projection, if you are looking eg North Sea east of 0 degrees ED 50 UTM 31N will do you right though google a UTM zone map to check you aren't in an area of the north sea where things get funny.
Thanks very much folks, I'll give your suggestions a wee shot this afternoon!
ED50 UTM31N isn't a projection that comes with Arc actually, you might therefore find it easier to project to WGS84 UTM31N rather than define your own projection, its up to you. The transformation you'll want to use from ED50 to WGS84 is number 18 out of the ones that will come up in the drop down list in the Project tool. I suggest you try using a geodatabase rather than shapefiles too - I far prefer them and when you are using a projected coordinate system they'll give you areas and lengths in the attribute data automatically.
I suggest you try using a geodatabase rather than shapefiles too
If I can get the DECC maps (e.g. oil and gas fields, UKCS grid etc) in that format, I'll try it out. Cheers.
they'll come in shapefile but you can import into geodatabase in ArcCatalog. I'd suggest having look at them in the future anyway, if you are going to be doing regular GIS work.
You've received some good and some 'interesting' advice here. To help you come up with the correct outcome here is what I recommend you do (I work in the GIS department of an offshore marine surveying company by the way and do this every week).
To export your ED50 file from DECC into a WGS84 projection (Geographic Coordinate Systems>World>WGS 1984.prj) follow bigjims instructions and set the Geographic Transformation to 'ED_1950_To_WGS_1984_18' in the appropriate box of the 'Project' tool. Transformation will work well for the N. Sea and works acceptably for the rest of the UK.
This will generate a new file 'stretched' over the WGS84 spheroid of the world rather than the one used by ED50. UTM is the projected projection system based on WGS84 so depending on where you are in the UK you can set the projection of your ArcMap project to either UTM30N or UTM31N and your DECC data will be in the right place. If you want to combine this with terrestrial data in a BNG projection heed bigjims advice about using the 'Petroleum' transformation.
If you aren't confident I would steer clear of geodatabases. They will import any file you tell it regardless of whether the projection and datum is suitable for the .gdb settings.
Goodluck!
DAVE
Right, turns out that the Project tool is one that I had used before, but with no success. However, I tried it again with the instructions above and it still does not stretch the map - it doesn't look like I've done anything to it at all!
Where are you looking at the projected data, in arccatalog preview or in arc map? If it is an existing Arcmap mxd then if you haven't changed the coordinate system to your new UTM one, it will still look the same, as it will just be reprojecting your UTM data 'on the fly' back to ED50. Either add your new projected shapefile to a new empty mxd, or change the coordinate system of your old mxd to WGS84 UTM31N or whatever you went with.
You'll get the hang of it eventually 
I am looking at it in ArcMap, by dragging the shapefile from ArcCatalog (also used the "Add Data" menu option) into a new mxd file.
Hmm if you right click>properties on your shapefile in Arccatalog, under the XY coordinate system tab is it displaying the correct UTM projection you defined when you reprojected? SOunds like it hasn't been projected if it is still looking squished (technical GIS term there)
I'm looking to learn more about ArcGIS to improve my prospects of getting a job in our GIS department. I have experience working with spatial data in my degree - something that I enjoyed - but never with ArcGIS.
Are there any courses anyone could recommend or would it be best to do an MSc in a related discipline?
Thanks and sorry to the OP for the hijack.
The ESRI courses are fantastic but expensive if your work isn't paying for them. Depending on what you want to do learning on the job does work, I did 6 months of GIS at uni and am now a full time jizzer, having come back into it through work. It means I'm not a die-hard GIS technical guru and I can't do fancy Python scripts, but I can do most things I come across in my line of work better than a fresh MSc graduate.
Cheers bigjim.
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