Mapping a Set of Points in ArcGIS

For this example, we shall use ArcGIS to map the points. First, we need to get the coordinates into a format which the GIS is capable of interpreting and importing. ArcGIS is actually capable of importing a range of file types but one of the most robust and reliable formats for importing a set of point coordinates is dBASE(IV) format. Our coordinates may be in a range of formats including a simple text file, a comma-delimited (.csv) file, or a tab-delimited (.tab) file. Coordinates may also have been processed or edited in other software packages such as Microsoft Excel and saved, for example, as an .xls file. One of the best ways to get our coordinates into dBASE format is to use Microsoft Access. Access can import data from all of the above formats.

In this example, our data are available as a text file crime_grefs.txt. You can download the file by clicking [here]. Microsoft Access should be used to provide a unique (ideally numeric) identifier for each point (e.g. by using the auto-numbering function in the import wizard), to name each field (preferably with a name of less than 8 characters and not using spaces, brackets or dashes), and to define the data type of each field (e.g. integer, text). Make sure you define the X and Y fields as numbers/ integers.

This creates a table with three fields: CRIMEID, X, Y.

This table can be exported in dBASE(IV) format by selecting the table, right-clicking and then choosing "Export". Specify a pathname and suitable filename e.g. "crimes", select Save as type dBASE IV (*.dbf) and then click "Export". This will create the file CRIMES.DBF (and, probably, some other associated files such as CRIMES.INF and CRIMES.MDX).

ArcGIS

Start ArcMap (with a new empty map). Select File > Add Data. Select your .dbf file and click "Add".

This adds your .dbf file as a Table. You can open the table by right-clicking on it in the Layers Frame and selecting "Open".

Now, we need to get ArcMap to use the coordinates from the table to create points which we can then map. We can do this using the Add XY data tool in ArcMap.

Select Tools > Add XY data. Specify that we wish to use CRIMES as the table from which the coordinates will come and then select the fields which contain the X and Y coordinates of the points (in this case, X and Y respectively). At this point, we can also specify the coordinate system and/or projection for the points. In this example, the coordinate system is the British National Grid. Select "Edit", then "Select" (to select a pre-existing coordinate system) and then browse until you find "British National Grid.prj" (under Projected Coordinate Systems > National Grids); click "Add" and then "OK". This gives ArcMap all of the information it needs to map the coordinates. Click "OK". ArcMap will automatically map the points in the Data Frame.

These features (points) are not yet permanent though. If you were to close ArcMap now, they would not be saved. To create a permanent dataset of the points, you need to convert the points to, for instance, a shapefile or a feature class. We will create a shapefile. Right-click on the CRIMES Events layer in the Layers frame and then select Data > Export Data. Choose to save all features; specify that you wish to use the same coordinate system as the layer's source data; browse to the relevant directory and provide a suitable filename e.g. car_crimes.shp (Note that because both dBASE and some ArcGIS files use the extension .dbf, things can get a little confusing! It is best therefore, to name the shapefile something different to the name you used for the .dbf file previously); leave the 'Save as type' as "shapefile"; click "Save"; and finally "OK". ArcMap will ask if you want to add the exported data as a map to the layer - say "Yes".

Your points are now mapped!

NB. To help you keep track of the files that you have created, it is probably worth tidying up and removing any files which you no longer need. You could therefore remove the "CRIMES Events" layer and the "CRIMES" table from your Map now by right-clicking on them and selecting "Remove".