Explore Dutch Morphodynamic history!
Many of the maps belonging to the handwritten archive of the Dutch bathymetry have been scanned during the digital revolution we are experiencing in the last decades. Digitizing maps will help not to lose the historical information. Moreover digital format is easier to store and to manage and it allows for a wider number of users. However, the job is not over yet. For example, unscanned maps of the Wadden Zee still exist.
In this tutorial the procedure to convert a map scan (.png file) in a shapefile with QGIS will be explained. The conversion could be useful for further uses of the bathymetry data contained in the maps, for instance they could be used as inputs for forecast and prediction models. In the context of Geospatial analysis, it would also be possible to process the shapefiles with GIS functionalities.
A map of the Wadden Zee Friesche Zeegat dated 1921 is used as example for this tutorial.
First of all we need to load the map: Layer > Add raster layer and choose the map you want to convert, then assign the preferred geographic coordinate system.
Now we need to create a new shapefile in which we will transfer the information contained in the map.
From the top menu go to Layer > Create layer > New Shapefile Layer:
We choose “line” as type because we want to reproduce the contour of the bathymetric isolines. In the menu, we can assign the CRS and we can also choose the field we want to add the shape file to. These fields will form the attribute table we could query to obtain information about the features. In addition to the ID field that is the default field, you should add the “depth” field in which we can write the depth value related to the bathymetric line.
We can now start editing the shape file. Select the shapefile and right click on it to select “Toggle editing” and begin the editing session. From the top menu you can choose “Add feature” to start drawing the lines by the left click of the mouse. You can also choose other options, for example “Add circular string” if you need to draw a circle. If you make a mistake or need to make changes you can move the features with “Move feature(s)” or the nodes with “Node tool”. When the line is complete right click on the last point to write the field parameters. Don’t forget to save.
Once you have finished editing your shapefile exit the editing session by clicking on the “Toggle editing” button again.
If we remove the underlying map, this is what we obtain.
The resulting shapefile contains a table of the isolines at different depths, High Water and Low Water levels.
