How to download and collect layers - BGS Example
Maps created using GIS are largely made up of combining layers of data from a variety of sources, and then selecting those parts which we want to display in our map or maps. Here we will look at collecting information from the British Geological Society website.
In order to be able to display areas of peat and alluvial sediments in our maps, we can make use of the free mapping datasets available from the British Geological Survey. The link is shown under the Resources section above, but is repeated here:-
You will see entries inviting you to choose the scale of map required, like this:
"DiGMapGB-50 - 1:50 000 scale data
"DiGMapGB-625 - 1:625 000 scale data
Select which scale you prefer. The 1:625, 000 scale is perfectly adequate for the regional maps which we wish to produce for East Anglia.
Having clicked on DigMapGB-625, you have another choice of file - 'Bedrock' or 'Superficial deposits'. We want 'Superficial deposits' which will include Peat and sediments. We also want to select ESRI, which tells us that the map will be delivered in ESRI shapefile format. You will see this entry:-
"DiGMapGB-625 data 1: 625 000 ESRI® [Superficial deposits]
Click on the down load now arrow to collect the downloaded map layers. You will have to decide where you wish to store the download, and once arrived, you will need to unzip the folder.
( In fact, QGIS has the cabability to load a shapefile from a zipped file, without unzipping it. But you cannot edit it from the zipped file, so it is more straight forward to unpack all your zipped QGIS files.)
Now we need to get hold of a style file for this data. (Otherwise we have to style it ourselves, which is a long job)
Luckily EDINA comes to our rescue here:-
Select the appropriate SLD file, in our case it is 1:625,000 Scale SLD.zip. Once downloaded, open the folder, and "extract all files" to unzip the files.
I would download the other SLD files for possible future use as well. SLD stands for styled layer descriptor. It is not the native styling method for QGIS, and is not always fully supported, especially in the most complex styles used by the Ordnance Survey. The QGIS preferred alternative is its own .QML file format for styles. So when given a choice of downloading an .sld file or a .qml, it is best to choose .qml. In this case we are only given .sld files, but these work fine in this instance.
Until we apply a style or styles to our data, QGIS will give it a simple fill, with a randomly selected colour. Here we see above that every polygon in the layer is coloured brown by QGIS. As this is a random allocation, your example will almost certainly be given a different colour.
To load the style file we have downloaded, proceed as follows:-
Double click on the Layer Name to open the properties dialogue. Select the Style tab. Down at the bottom left hand side of the style tab, you will see a button marked'Style' containing a small down arrow. Click on this arrow to get a drop down menu.
Select 'Load style...' and then browse to where you have saved the .sld files. If the folder appears blank, make sure that the filter button at bottom right is changed from 'QGIS Layer Style Fill (*.qml)' to 'SLD file (.sld)'
Seect the file marked, "gb_625k_superficial_geology_polygons.sld", and click 'Open'.
Once you have done this, you will see that the 'Simple fill' has been replaced by a 'Rule-based' style with an array of 14 different colours. Click Apply and then OK and the map should look like this.
In order to clip off the part we want, we have to create a new layer, containing a new 'polygon' which will be used as the "scissors" to cut off the material we wish to keep.
The process begins by selecting: Layer/Create Layer/New Shapefile Layer.
This will open a dialogue entitled, 'New Vector Layer' which requires some input from you. In the 'Type' section, select 'polygon'. Check that the Coordinate Reference System drop down menu says, 'EPSG:27700, OSGB 1936 / British National Grid.
Ignore the rest and click OK.
A new dialogue opens which requires you to give your new file a name, such as 'cliplayer', and decide where you want it stored.
The name of the layer will appear in the list of layers. We now need to draw our polygon around East Anglia. Click on the icon of a pencil, which turns on the editing feature, and makes the appropriate tools available to us. Find the 'Add feature' button, and click it.
The next bit is tricky. Move the cursor to the top left corner of the area which you want to include. When you are happy left click, and a dot appears on the map of England. Release your click and move the cursor to the top right of the rectangle you require. A string of dots should follow your cursor until you left click on the new point. A line should appear between the two points. Now release that click and move down to the bottom right corner and click again. You should now see a red triangle outlined. Release the click and move to your final point and left click to complete the feature. You must now right click to release the cursor. You will be asked to ID the feature, but you can just click OK. The new polygon should now be a filled square on the map.
Cick on the pencil icon to turn off editing, and you will be asked if you wish to save the new feature, and answer 'Yes' if you are happy or 'No' to have a repeat performance. Here I would also normally change the style of the clip layer to be transparent, as follows:-
Double click on the Layer Name to open the properties dialogue. Select the Style tab. Click on coloured block under 'Simple Fill'. Select the Colours drop down menu and click on Transparent Fill. Finally we come to actual clipping operation.
(Note that this clip layer can be used repeatedly on each of our layers that we want to use in our final production.)
Select 'Vector/Geoprocessing tools/clip....'. A new dialogue appears which requires you to specify 'Input vector layer', which is the name of the layer you wish to clip. Select from the drop down menu. The 'clip layer' is the name you gave to the rectangle you drew to specify the area you wanted. Select it from the drop down menu.
Finally, you also need to name the new layer, or 'Output shapefile' which you wish to create. Do this by browsing to the folder where it is to be stored, and enter 'digmap625_eastanglia' or similar, and click 'Save'. This returns you to the clip dialogue, where it should be checked in the box 'Add result to canvas'. After clicking OK it should appear on your map. Now close the clip dialogue.
Now we have a new layer showing East Anglia's surface geology, but once again it is styled in QGIS default of a simple fill. So once again, load the stylefile marked, "gb_625k_superficial_geology_polygons.sld", and click 'Open'. The map of East Anglia should now be properly coloured up again
The simplest way to isolate the areas of peat and alluvium is to turn off the styles for all the polygons which are not in these two classes. Look for the name of the layer you just created in the 'Layers' window on the left of the program screen. Next to the check mark, (always x in Windows, may be a tick in Linux), is a tiny + or plus sign. Click on this to reveal all the styling information. Every class of colour is checked. Now uncheck everything except 'ALV-CLSS' and 'PEAT-PEAT'.
You should now see only areas of yellow alluvium and brown peat. Note that there is no clear coastline. In order to show the coast we will need to find another layer of coast, and a good source will be the OS OpenData Strategi file.
Our map above does show the alluvium and peat features which we want to display as part of a topographic map created in the manner of H C Darby's "Domesday Geography of Eastern England", and later authors such as Dr Sam Newton. However it will be a lot tidier to create a new layer which consists solely of the two features without any other geological features.
We can do this by making use of the attribute table and the 'Select by expression' feature, which is demonstrated in the next section.
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This page created 4th October 2015|
Last updated 10th October 2015.
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