Introduction to Digimap
Digimap is a website that allows you to download Ordnance Survey maps. The site describes itself as follows:
“Digimap is a collection of EDINA services that deliver maps and map data of Great Britain to UK tertiary education. Data is available either to download to use with appropriate application software such as GIS or CAD, or as maps generated by Digimap online.”
Digimap is a fantastic resource that allows students free access to data that would otherwise cost hundreds of pounds. Please make good use of it!
How do I get to Digimap?
Access to Digimap data is a bit long-winded but don't be put off, it's well worth the effort. The best way to get to Digimap is via the Student Portal. That way, your login details are carried with you and this makes the login process much easier.
Once you are logged in to the student portal, click the tab that says "Library & IT". Next, find the e-Library box and under Online Resources, click the link that says "Electronic databases, electronic journals and e-books". This will take you to a list of all the electronic data you have access to as a student of the University.
Scroll down the list until you find the link that says "Digimap: Ordnance Survey Collection" and click it. A new browser window will open and the EDINA Digimap login page will load. Click the "Login" button (via UK federation). You are taken to a page at The UK Access Management Federation site. Select "University of Greenwich" from the drop-down list, leave the other option set to "Remember for session" and click the "Select" button. Your credentials are checked against a list for the University and if all is well, you will be taken to the Digimap homepage.
If you have never used Digimap before, you may need to register. This is a once-only process but it can take up to 48 hours for your registration to be verified.
How do I download a raster map?
The Digimap homepage offers 4 main options, of which only the first 2; Digimap - Ordnance Survey Collection and Historic Digimap are currently available. Click the option at the top for the Ordnance Survey Collection.
The next page you see confirms your name and displays a copyright notice. This copyright notice will be displayed each time you login to Digimap and it is a reminder that the data may only be used for educational purposes.
"Use of Ordnance Survey data from Digimap is strictly limited to Educational Purposes only."
Click the green "I agree…" button to proceed to the data selection page.
There are a number of options for building maps and downloading map tiles on this page. Let's assume you would like to download an Ordnance Survey 1:25,000 raster map for use as a map base for a diagram you are creating in Photoshop. Proceed as follows:
Click the link that says "Data Download Services" at the bottom of the page. On the next page there are 5 options. Click "Data Download". The next page shows you the 9 types of data available for download. You must select the data type you require. Click the radio button that says "TIFF (LZW Compressed)" in the box entitled OS 1:25000 Scale Colour Raster and then click the "Continue" button.
You must now select the area you want. Decide what search criteria you would like to use (place name, postcode, grid reference etc.) and enter a search term in the appropriate box. Let's assume we want a map of Avery Hill. Select the "Place name" radio button and enter "Avery Hill" as the search term. Click the "Search" button to continue to the next stage.
On the next page, you are presented with a small-scale map with the place you searched for at its centre and on the right, a list of available tiles. Each tile is a square section of map and you will need to identify which tile or tiles are required for your purposes. Probably the best way to start is to zoom in to the map using the tools provided. As you zoom in, the map will show more deytail and the number of tiles covering the area will reduce. If you look carefully at the map, you will see that the tile boundaries are shown so that you can see what is included. If you continue zooming until you can see the detail of the Avery Hill campus, you will notice that there is only one tile needed - tq47. Select "tq47" in the tile list and click the "Continue" button.
Finally, you are taken to the download page. If you have followed this example, you will see that you will be downloading 1 tile (tq47) and that the file size is approximately 8MB. There are also a number of compression options available but, the default (zip archive) is the best for general use.
Note the warning about the time required to make the data available (it can take a while, especially at busy periods) and click the "Extract Data" button. The next page is a holding page and just tells you that your data is being retrieved and offers a link to check the status of your request. Click the link now. If the Digimap service is very busy, you may have to wait a little longer but in most cases, you will see a page that contains a list of files ready for download. The only file you need is the one called data.zip because all the other files in the list are also contained in the zip file.
Download the file by clicking on the file name in the list. You will get a little Windows dialogue box asking if you want to save the file. Click the "Save" button and select a folder where the file will be saved (remember where you save the file, you will need to find it later).
How do I extract the map?
Before you can open the TIFF file in Photoshop, you must extract it from the archive. The method for doing this may vary depending on your zip software but you can usually just double-click data.zip in order to open it. In this example, I am using WinZip to open the archive and you can see that there are 4 files. In fact, you only need one of them. Click-and-drag tq47.tif into the folder where you keep your project files.
How do I work with the map in Photoshop?
Start Photoshop and open tq47.tip (). The first thing you may notice is that the image looks crude and blocky. That's because the OS saves raster maps in Index Color mode in order to keep file sizes down. That means the maps use a restricted colour palette of just 256 colours and you will find that many Photoshop tools are not available (greyed-out) when working with index colour images. In order to work with this map in Photoshop, you will need to change the mode to RGB Color. To do this, select from the pull-down menu. Once you have done this, you will see an immediate improvement in the quality of the image and you will have access to all the Photoshop tools such as layers.
You can now use the map just as you would any other image or photograph.
Printing and scale considerations
The OS 1:25000 raster maps have a print resolution of 100 pixels per centimetre. This information will help when working out how big your map will be when it's printed. For example, if you want to print the whole tile (4,000 x 4,000 pixels), the resulting print will be 40cm x 40cm. This is too big for an A3 sheet but will fit comfortably on an A1 sheet.
If you need to reduce the size of the map in order to fit an A3 sheet, you could crop the map (maintaining its scale) so that it occupies less space or you could reduce the image size to 50% (in Photoshop). This will effectively change the print scale to 1:50,000 and the printed map will be 20cm x 20cm.
Always leave enough whitespace at the right-hand edge or along the bottom of the sheet for a legend and title block. Also, remember to include a scale bar and north point when presenting map extracts even if the scale and orientation seem obvious.
Top Tip: Make sure any text you add to the map and legend is easily readable at the print scale.