Creating a GIS layer of major watercourses

I have been thinking this week about producing a GIS layer of major (modern) watercourses for use in our analysis.  The difficulty with this is that most data sources will provide far too much detail in this regard.  The two OS OpenData products with hydrological data in them are VectorMap and Meridian 2, both of which map a great many watercourses across the country.  This is all well and good when examining data in detail, but is rather confusing when looking at the bigger picture:

Input data
The Meridian 2 rivers layer

Fortunately, there is a simple method by which we can extract only important watercourses from the Meridian 2 data.  The data features a “Name” field, which records the name of the watercourses plotted, but this is (usefully to my purpose) only applied in the case of major watercourses.  Therefore, we can use the attribute select tool to select all of the features in the dataset with a name (the query is: NOT “NAME” = ”):

Stage 1
Selecting all items with a name

However, this still includes canals, which are not of interest to those of us who are not working on post-medieval archaeology.  Therefore, we change the option in the attribute select tool from “Create new selection” to “Remove from current selection” and deselect features with the word “Canal” in their name (the query is: “NAME” LIKE ‘%Canal%’):

Stage 2
And then deselecting all items with "Canal" in their name

We can then export these selected features to a new layer, which is the basis of our major watercourses layer.  There is one small difficulty that arises in that many small sections of interlinking watercourses do not appear to be named in Meridian 2.  I therefore spent a couple of hours working through the new layer and drawing these back in using the edit tools in ArcGIS, based upon the data in the original layer, in order to produce a map that looked more like a proper river network.  I also deleted some of the straight watercourses left in The Fens where they clearly did not take the place of previous natural watercourses.  I only did this cleaning task for England (due to the remit of our project), but it would not take long to do for the country as a whole.  Incidentally, some of these gaps were caused by there being reservoirs along the course of the rivers in question, which could easily be plotted on the map using the relevant Meridian 2 layer*: these were not plotted for our purposes due to being modern features.

The final result was as follows:

Output data
The result!

Clearly, the result is not perfect, but I think it should serve its purpose quite well.  Obviously, the biggest issue with this map from an archaeological perspective is that these are modern watercourses, which clearly do not in all cases have a direct equivalence to the form of past watercourses: one very obvious area where this is the case is in The Fens.  The more recent your period of interest, the less problematic this is (as things will have changed less), but it is worth keeping in mind even for more modern archaeological work.  Creating a map of ancient watercourses is a much less simple task…

Chris Green

* A process similar to this would not work as well for the lakes / reservoirs layer in Meridian 2, simply due to the fact that most reservoirs are not called “Something Reservoir”, but given more bucolic names like “Draycote Water”.  As such, it would be much harder to extract these modern features from the dataset than it was with canals.

Author: Chris Green

Postdoctoral Researcher (GIS)

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