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Map Room Home page | |
written by Nigel
James
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MAPINFO How to... | |
No. 6: Create a basic point data map using Ordnance Survey grid references | |
(References to tables, folders etc., are not applicable outside the Map Room, so if you are located elsewhere, you should substitute your own) | |
This explains how to use MapInfo to plot point data using OS grid references. MapInfo cannot read OS grid references directly, so they have to be converted to x,y coordinates in metres. You can do this using the Conversion Tools* in MapInfo. Whilst this How to explains the method of creating a data table in MapInfo, you can also prepare your table using a program such as Excel and import it into MapInfo. See MapInfo How to No. 12 for more information. As this is a worked example only, you can substitute your own grid refs and other data if desired. |
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*The MapInfo tool Conversion Tools, referred to in this guide, can be downloaded from the geo tools section of the Map Room. | |
1. Start MapInfo. 2. Create a new data table by opening the File menu and clicking New Table. In the New Table dialog, Open New Browser should be ticked and Open New Mapper unticked. Click Create. 3. You will now see the New Table Structure Dialog. Our table will have two fields initially: Grid_ref and Name. (Note that field names cannot have spaces, so use an underscore to join the words). 4. In the Field Information group, enter the field name as Grid_Ref. Leave type as character and set the width as 12 (the width must be at least the number of characters in the longest grid ref - NGConvert can handle GRs with two letters and up to 10 numbers). 5. Click the Add Field button to enter the next field. This will be the placename. 6 .Enter the name as Name and select Character from the type list (again, use a field width sufficient for the longest name). 7. The structure is now complete, so click Create. Save the table as Grid_refs in your user folder. 8. Open your table. It will appear as a single row table (just the field names). 9. Now add some rows to the table by holding down the Control key and pressing E for each row required. This example uses four grid refs so create four rows (you can add further rows at any time). If the rows do not appear, make the window larger and push the scroll button back to the top of the window. 10. Now we can enter the data. Enter the following in the four rows (grid ref and name in separate fields!) SP 5106 Oxford Note that the grid refs can be in upper or lower case, or a mixture and grid refs can be 2 to 10 digits - Conversion tools can handle this, including spaces in the references). 11. When you have entered the data, save the table again. 12. With the table displayed in a browser, open the Conversion tools menu and click Table of OS grid refs to metres. This will display the dialog: |
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All open tables are listed in the Select Table list. 13. In the Select Table list, click your table, which will then add the existing columns to the other lists. Click Grid_Ref in the OSNGR Column list as this contains the grid references and choose a symbol by clicking the Symbol button |
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14. To convert the references, click OK. 15. The grid references will be converted and the eastings and northings stored in two new columns in your table. A symbol will be created for each record. |
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If any errors occured then the eastings and northings will be set to 0.0 You can then easily spot any errors. The usual errors are invalid prefix letters and/or an odd number of digits. 16. Now you can display your points on a map. Open the File menu and click Open Table. We will use the UK from the Euromaps data set in this example, so open the Euromaps folder on drive D. Next, open the table overview.tab. Zoom in to display The UK only. 20. With the UK displayed, open the Map menu and click Options. Click the Projection button and in the Choose projection dialog, select British Coordinate Systems in the Category list and British National Grid in the Category members list. Click OK to re-project the map on the OSNG projection. (This is not strctly necessary as MapInfo will reprojection the points to whichever projection you are using, but it will display quicker if you set the map projection first). 21. You can label the places with their names by opening the Layer Control and ticking the Autolabel box for the Grid_refs layer. Next, click the Label button and check that Name is selected in the label box. Click OK in each dialog until the map is redisplayed with the names. 22. You could instead label the places with their grid references, by opening the Layer control and clicking the label button, then selecting Grid_ref for the label instead. 24. Once you have created a points table and mapped it, you may wish to add further points. Although you can add them to the data table, they will not be added to the map automatically, as you will have to run Conversion tools again to convert the new records and add symbols. |
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This is a brief and specific guide to this topic, for more general information, see: MapInfo - an easy guide for new users | |
Nigel
James
Bodleian Library 2003 |