County Durham SMR
This first set of exercises in the use of OS data and GIS is based on information from the County Durham Sites and Monuments Record.
Open a new View, and add the Theme distpoly.shp. This is a map of County Durham showing the boundaries of the various districts, along with the Unitary Authority of Darlington. The boundaries are taken from the Meridian data set.
Other shapefiles drawn from OS Strategi data will give an idea of the geography of the county:
- Roads: mway.shp, primary.shp, aroad.shp, broad.shp, minor.shp
- Rivers: dur_river.shp
- Main towns: main_towns.shp
From the distribution of the main towns and roads it can be seen that population density varies considerably across the county; the north and east being most densely settled, while Wear Valley and Teesdale are relatively thinly-populated and contain substantial areas of upland.
This first exercise uses a set of data consisting of archaeological sites in Easington District. Adjust the View so that the size of this area is maximised.
Add the Theme easington.shp and display it. One thing which is immediately obvious is that the site locations conform to a grid pattern. This is of course inevitable, since the nature of grid references is to give a location to a corner of a square within which the point of interest lies, but here the fact is extremely noticeable. The site locations are in fact those which can be obtained from the SMR website [which can be reached via Durham County Council Information service, http://www.durham.gov.uk/durhamcc/usp.nsf], and are given to four figures (e.g. NZ1234), which means that the points fall at 1000m intervals. If you are looking at just one site, or a group which are widely separated, it's easy to forget the real resolution of your data set. Published grid references are often given to four figures, sometimes because better information isn't available, but also for reasons of archaeological sensitivity or to protect the privacy of property owners.
If you open the Attribute Table for the shapefile easington.shp you can see that there are really three fields which contain archaeologically interesting information:
name is the present-day name of the site
period contains a code for the age of the site, in broad categories:
1p prehistoric
2r roman
3em early medieval
4m medieval
5pm post-medieval
6mo modern
7u unknown
type indicates the nature of the site, 'enclosure', 'chapel', 'flints' etc. In all, there are 49 types of site, from 'arrowhead' to 'winding engine house'.
An obvious thing to do to increase the information content of the current View would be to change the symbol used for each site to reflect its age. This should work well, since there are only seven periods, and it will be possible to create seven symbols which vary in shape and colour sufficiently to be easily distinguishable from each other, but it might not be a useful way to distinguish one type of site from another, because there are so many different types that creating enough symbols which are obviously different might be impossible.
Open the Legend Editor for the Easington theme, and change the Legend Type from Single Symbol to Unique Value; in the Classification select the appropriate field from the attribute table (Period), and a series of new symbols will appear, with information showing how they will be applied to the Theme. Initially each symbol will be of the same kind (a round dot), and the same size, but a different colour. However, these will be chosen from the default colour palette ('Bountiful Harvest') which has a very limited range. Click on the Apply button to see the effect. You can do much better than this by taking control of the symbols yourself.
If you double-click on the symbol for 1p, the Symbol Window will appear. This gives you access to a range of tools for changing the attributes of any kind of symbol (point, line, or polygon). The marker palette is open first, shown by the third button on the toolbar; this allows you to change the shape and size of point marker. The fifth button gives access to the colour palette. For this symbol just change the colour. It's probably best to select colours which are obviously different from each other - e.g. red, blue, green, black and so on, rather than shades of the same or similar colours. Use red for this one. Having chosen the colour, click the mouse back in the Legend Editor window (the symbol shown here will have changed by now, though the View remains unchanged) and double-click on the 2r symbol. This time change the shape to a square and the colour to black.
For the 3em symbol, use a square which has an outline and fill of different colours. When you click on the colour button, use the field Color to set the foreground to a light blue and the outline to a contrasting colour (the default is black, and that is probably the best one for this purpose).
Treat the other symbols similarly; the following symbols give a reasonable result:
4m square, black outline, darker blue fill
5pm triangle, pink fill, black outline
6mo triangle, red fill, black outline
7u open circle, black outline
(It may be a good idea to increase the size of the triangle symbols from the default 8 point to 10.)
Before applying this scheme, change the labels which will be shown for each period in the legend. Just click the mouse in the Label field and replace the existing text (which will be the same as the code in the Attribute Table) with something more immediately meaningful - 'prehistoric' instead of '1p' etc. Finally click on Apply.
If you have chosen symbols as indicated above, your View should look like this:
You may notice a number of apparent anomalies. At the extreme south-east are two symbols which appear to fall outside the area of Easington District. Using the Identify button the modern period site can be seen to be a reference to an archaeological assessment of the coastline called 'Turning the Tide', while the prehistoric one is the chipping-floor site at Crimdon Dean. A small section of 1000m grid has been created as the shapefile km_grid.shp; if you add this to the View you can see the explanation for the location of these points 'outside' the District. Almost half the 1000m square defined by the grid reference given to the Crimdon site (NZ4836) lies within Easington, and this is where the site actually is, while NZ4936 gives room for the point indicating the assessment to be in the sea just off the coast, reasonably so since it refers to an investigation at a number of places along the coast.
To the west, the find-spot of a quern at Littletown is also placed outside the District, but again this apparent anomaly is caused by the low resolution of the four-figure references.
In a small number of cases two points share a location (e.g. the lime kiln at Crimdon and the chipping-floor at Blackhall Rocks, or the Londonderry Institute and the coin find-spot at Seaham). This is always a possibility, and the lower the resolution of the grid references, the more likely it is to happen. It's one of the reasons why it's often not practicable to show all of your information on one map. Even if sites don't share identical locations, they may be so close that their symbols overlap; that doesn't seem to be a problem looking at just the Easington area, but if you zoom out so that the whole county can be seen the symbols do look quite cluttered.
The situation becomes worse if you want to add text to the View. In order to add names to these sites, make sure that the easington.shp Theme is active. Click on Theme - Properties and in the window which opens choose the Text Labels option. In the Label Field box check that Name is showing, and if not choose it from the scrollable list. Under this field there is a diagram which allows you to choose where the label will appear relative to the point, the default being to the left; leave this as it is.
Now use Theme - Auto - label. Accept the default option of Find Best Label Placement, and click OK. The placement of labels will be reasonably satisfactory, and all the labels should be readable, but in order to achieve this quite a number of sites won't have been labelled at all. Erase the labels using Edit - Select All Graphics, followed by the Delete key (on the keyboard). Now try labelling the sites again, but this time check the box labelled Allow overlapping labels. The result is that all the sites are labelled, but in contrasting colours where they overlap. This is neither aesthetically pleasing nor very readable. Erase these graphics as well, and try the final auto-label option, which is to Use Theme's Text Label Placement Property. This is even worse, with all labels written to the left of the point, and overlapping to such an extent that very few can be read.
When auto-labelling points, it's worth noting the existence of a check-box which is itself labelled Scale Labels. This is checked by default, and its effect is to fix the size of the label text relative to the map, so that if you subsequently zoom in or out the text will become progressively larger or smaller. If you un-check the box the text remains the same size, irrespective of the scale of the map.
When you need to add text to a map of archaeological sites, unless you have a very small number of points you will almost always have to look at subsets of your data, either limiting the geographical area or choosing some categories of site and not others.
Selecting subsets of your data using their non-spatial attributes (such as site type) may be accomplished by using Theme - Properties, and selecting the Definition option. In the window you will find a button with an icon of a hammer and a question mark (the 'query builder'); click on this.
In the new window the fields of the Attribute Table appear down the left-hand side, while on the right is an empty list called Values. In between are a series of buttons labelled with logical functions. There is also a check-box called Update Values; make sure that there is a tick in this box. At the bottom is a blank field where the query will be written.
Suppose you want to select all the deserted medieval villages. In the list of Fields, double-click on the word [Type]; this should enter the following character string into the previously-blank field:
([Type])
The 'Values' list should also show all the available entries in this field. Now press the '=' button, and scroll down the list until you find "Deserted Medieval Village". In fact there are two choices here, "Deserted Medieval Village, Village" and "Deserted Medieval Village, Shrunken Medieval Village, Village", and if you scroll on down the list you'll find "Shrunken Medieval Village, Village" as well. We will need to select all these. (This is a simple but useful indication of the need to understand the nature of your data set before you start to perform any query operations.)
At the moment the query field should show:
([Type] = )
Double-click on "Deserted Medieval Village, Shrunken Medieval Village, Village", so that you see this in the query field:
( [Type] = "Deserted Medieval Village, Shrunken Medieval Village, Village" )
(The text will have wrapped itself within the field, probably breaking the line after 'Shrunken'). This has selected one of the possibilities. The next step often causes problems for people who aren't used to creating database queries of this sort.
Click (once) on the or button, then double-click on [Type], click once on '=', double-click on "Deserted Medieval Village, Village", click once on or, double-click on [Type], click once on '=', and finally double-click on "Shrunken Medieval Village, Village". By now the query should look like this:
( [Type] = "Deserted Medieval Village,Shrunken Medieval Village, Village") or ([Type] = "Deserted Medieval Village, Village") or ([Type] = "Shrunken Medieval Village, Village" )
Clicking OK in this window and in the Theme Properties window should show you a set of ten points in the View.
Two problems often occur when users are unfamiliar with the software. One is to create a query like this:
( [Type] = "Deserted Medieval Village,Shrunken Medieval Village, Village" or "Deserted Medieval Village, Village" or "Shrunken Medieval Village, Village" )
which makes human but not logical sense, and will cause an error message, or one like this:
( [Type] = "Deserted Medieval Village, Shrunken Medieval Village, Village") and ([Type] = "Deserted Medieval Village, Village") and ([Type] = "Shrunken Medieval Village, Village" )
which won't generate an error message, but won't select any sites either. In a human sense, again, we do want to select each of these categories of site, so the use of the word 'and' seems appropriate to many people, but what we are doing is to construct a test which will be applied to each site in turn, and since each has only one entry in its 'Type' field they will all fail the test and no sites will be selected.
Assuming that the selection has worked, now try using the Autolabel function again, first of all with the default Find Best Label Placement option. Even with the reduced number of sites, the result may still not be perfect; depending on the size of the View window on the screen, and some sites may still be without a label, unless you use the option to Allow Overlapping Labels. In the latter case the labels which do overlap will appear in a different colour from the rest, and now that we are dealing with a small number of sites it's practicable to use the pointer tool (the black arrow next to the Identify tool) to select and move these so that they no longer interfere with other text. You will have to change their colour manually, though, using the commands Window - Show Symbol Window.
Even this may not produce an entirely satisfactory result, as in many cases you may find that it isn't possible to put all the labels in a position which makes it clear to which site they refer. The three sites of White Hurworth, Black Hurworth (both at Wingate) and Sheraton may fall into this category, depending on the size of the window in the View or Layout, and the size of text you want to use. A solution is to delete the automatically-placed labels and use one of the manual label tools, either the Callout Label, which puts the text in a box connected by a spike to the point to which it refers, or the Bullet Leader Label, which more simply connects the text and the point with an arrow.
If this selection of sites was to be used on more than one occasion, it would be worth saving it as an independent shapefile, rather than having to create a query each time, and this is very simply achieved by using the commands Theme - Convert to shapefile.