Drainage Basin Geomorphology

Determining Slope and Drainage Area of Streams

MapInfo Tutorial

A number of elements of drainage basin geomorphology require quantitative analysis of terrain attributes. Many students will be familiar with the idea of using pieces of string to estimate stream lengths from maps, or counting graph paper squares to estimate areas. These are tried and trusted techniques, but they are somewhat inaccurate and moreover can be very time consuming and tedious. For these reasons it is helpful to acquire some basic GIS skills that might be useful for automating basic terrain analysis tasks. In this practical you are required to measure the up-basin contributing drainage area and local slope at a number of channel heads. The estimation of drainage area additionally requires the user to digitise catchment boundaries (watersheds). In order to do this on a computer screen, you can use the MapInfo GIS package which is available on any ISS workstation and data that we have inputted from the Edina Digimap service. Most of you will be familiar with Digimap data already, so we hope that this practical might encourage you to use MapInfo (or similar GIS applications) in the future, to process your own mapping data acquired through Digimap. The key steps involved in the process are listed below:

  1. > Copying data files to your filestore (or onto a memory stick)

  2. > Launching the MapInfo application program

  3. > Starting up a MapInfo file

  4. > Navigating around a MapInfo map

  5. > Controlling multiple map layers

  6. > Measuring drainage areas

  7. > Modifying polygon data

  8. > Measuring and digitising linear features

  9. > Saving your work and ending MapInfo

1. Copying Data Files

To download the map data which has been compressed as a zip file for this tutorial, right-click (move the mouse cursor and hold down the right mouse button) here and then choose "Save Target As..." from the command list as in the figure below.

A standard "Save As" dialog box should appear, navigate to a folder where you would like to store the zip file, e.g. your network filestore or a memory stick. About 2 Mb disk space will be required. The name of the file should be "OLA5 Data.zip" by default. Re-type the filename in the "File name:" box if the box is blank. After the folder and filename are correctly chosen, click "Save" to download the file.

In the "Download complete" dialog box, click "Open" to view and then extract the content of the zip file.

To extract the MapInfo files, click "Extract all files" (in the Folder Tasks section) to start the "Extraction Wizard". You will need to click "Next >", then "Next >" again and then finally "Finish" to complete the process. All the files to work with MapInfo for this tutorial should now be stored in a subfolder entitled "OLA5 Data" under the folder where the zip file is originally saved. For example, if you have saved the zip file to "H:\My Documents", the full file path to the MapInfo files will be "H:\My Documents\OLA5 Data". Please note that you will only need to go through this file saving step one time.

Please also note that if you are working on an ISS workstation, anything saved to "C:\" will be removed as soon as you logged off from the computer. To avoid loosing any work, you should always work from your home filestore (which is normally linked to the "My Documents" folder).

2. Launching MapInfo Professional

The MapInfo application program is available via the "Start" menu >> "Your School Software" >> "Geography" >> "MapInfo" from any ISS workstation.

3. Starting up a MapInfo File

The mapped data that you find in MapInfo is stored in different tables (.tab files) which make up different map layers. Each layer can be viewed separately in different browsers, which can be viewed separately (in what are called Mappers) or together. In this case a number of layers have been put together for you and are saved into a workspace (.wor) so that when you open them you will have all that you need.

Choose “Open a Workspace” in the "Quick Start" dialog box. . You will need to navigate to the folder where the OLA5 data files are previously stored in the last step e.g. H:\My Documents\OLA5 Data. Click "lake district.wor" from the list and then "Open" to open the file. You should see a square map of an area near Haweswater in the Lake District.

4. Navigating around a MapInfo Map

Useful tools in this step: Select (), Zoom-in (), Zoom-out (), Grabber or Pan (), Change View ()

A multi-layered map should now appear in the centre of the window. To the right there should be a variety of MapInfo tools grouped into three categories of "Main", "Drawing" and "Web Services". You can drag and move any toolbar in order to see all the tools available. To explore this map, use the "Zoom-in" and "Zoom-out" tools in the "Main" toolbar. The hand symbol ("Grabber" tool) let users pan around the map while the "Change View" tool allows them to specify the map window width, scale and centre.

TIPS: If at any point you get lost (which can happen if you are zooming in and out) then there are a couple of easy ways of reviewing the map. If you right click anywhere in the window you are given the options of

  • Previous View : which is as it sounds and takes you back to where you were before (this is a good option if you accidentally move or zoom in or out)
  • View Entire Layer...: which allows you to pick one of the layers and will centre you back out to see all of the area

5. Working with Multiple Map Layers

Useful tool in this step: Layer Control ()

Although there should be little occasion for you visit the layer control in this practical it is worth understanding a little about how MapInfo functions. Within this practical there are four different layers presented to you.

Table 1: The four layers in lake district.wor
Lake_measurement This layer has been created for you to work on
Lakes_stream_info This is information that has been digitised for you; this includes an indication of the catchment boundaries, and markers to highlight those streams that we want you to investigate
Lake_District_contours This is OS land-form Profile data that has been downloaded from the Edina Digimap service
Lakes_land This is OS landline data that has been downloaded from Edina Digimap

The following image shows the layer control. Here you can see the four layers with three main checkboxes. (The fourth column of checkboxes is for auto labelling and can be ignored). The others indicate what you are permitted to do to each label. The “eye” indicates that you can view the layers; the arrow and star means that the features are selectable. The pencil means that the layer is editable. It is recommended in this practical that you only edit the Lake_measurement layer as this has been created for you for this purpose and it means that you will not alter the main base map layers. It is for this reason that this is the only layer that is editable (NB: you are only permitted to make one layer editable at one time).

To access this layer settings screen, right click anywhere on the map and then select "Layer Control". Alternatively, you can click the "Layer Control" button on the "Main" toolbar. Make sure the the layer settings are the same as the above diagram, click "OK" to proceed.

6. Measuring Drainage Areas

Useful tools in this step: Select (), Polygon ()

The first required feature is to estimate the drainage areas of the contributing areas upslope of each of the identified channel heads. It is first necessary to define these drainage areas. This can be done by digitally drawing the watershed boundaries for each of the contributing areas on your map layer (Lake_measurement). This process is known as digitising.

To do this you will need to use the "Polygon" tool on the "Drawing" toolbar. When depressed your cursor changes to a small cross. You need to left click once where you want to begin digitising your area and then you can effectively digitally “draw” where you want your drainage areas boundary. Each time you want to change direction, if you click the digital line will remain in that position and allow you to move on in a different direction. There are two ways to complete the polygon; either by pressing "Esc" or by double clicking the left mouse button. Both of these actions will complete the polygon and create an area on the layer.

If once you have completed your polygon you are not happy with the area, you have two options:

  • Option 1: To delete and start again. To do this you have to select the polygon, using the "Select" tool and then press the delete on your keyboard. (NB: to be able to delete you cannot have the "Reshape" tool switched on).
  • Option 2: To modify the existing polygon that you have drawn. The procedures are outlined in the next step. It is important to understand that you should digitise the drainage boundaries as accurately as possible (not always an easy task!)

7. Modifying Polygon Data

Useful tools in this step: Select (), Reshape (), Add Node ()

To change a polygon that you have created is relatively straightforward. Firstly you need to select the polygon you wish to alter by using the "Select" tool. Then you need to show the different nodes of the polygon (nodes are the points that you created by clicking and changing direction). These can be shown by switching on the "Reshape" tool from the "Drawing" toolbar. This then allows you to change the position of any of the nodes. If you wish to change a line to a greater degree the "Add Node" tool allows you to put in extra nodes in order to reshape the lines that you have drawn. Additionally, it is possible to delete nodes by selecting them with the "Select" tool and then pressing the delete key on the keyboard.

Once you have digitised the boundary of your drainage area, finding out the actual area of this polygon (and hence the drainage area) is very straightforward. Select the polygon using the "Select" tool then right click and select the "Get Info..." function shown as below:

This will then bring up some characteristics of this polygon as shown in the image below. One of these measurements is the total area of the polygon in squared kilometres. Notice that the great advantage of using MapInfo (or similar GIS) is that, unlike counting up squares of graph paper, any scaling adjustments required to account for the scale of the source mapping data are already automatically accounted for. MapInfo’s measurements are also very precise, which is why it is important to ensure that the digitised polygon is accurately representing the real boundaries of the contributing area.

8. Measuring & Digitising Linear Features

The second aspect required is the slope of these headwaters. This can be achieved by measuring the channel head of the stream, using the process described below.

Useful tools in this step : Select (), Zoom in () and out (), Pan (), Ruler ()

MapInfo permits you to measure linear features on the map, therefore this can be used to calculate the slope of the headwater of a stream (Remember: the contour lines on the 1:10000 scale maps used in this practical are set at a vertical spacing of 10m and can measure the distance between these).

Onscreen Map Measurement

Activate the measurement mode by clicking the "Ruler" button on the "Main" toolbar. Click at the intersection of the river with the contour line that is closest downstream to the channel head. A Ruler window will appear showing the segment length (Distance = ...) and cumulative distance (Total = ...) of the digitised path. Working upstream from this point, project a line through the channel head location and then project this line on to the intersection of the closest contour line upslope of the channel head. The Distance and Total (= total distance) values in the Ruler Dialog Box update every time the screen cursor is moved. To avoid any error at the closing point, place the cursor over there but without clicking. Press the ESC button to freeze the measurement values. This will also leave the measuring mode so the cursor movement should no longer affect the measurement values. The local slope gradient at the channel head location can then be estimated by dividing the contour interval (i.e. 10m) by the total distance (in metres).

Now you can start to digitise another stream. If necessary, use the zooming and panning tools to change the map display.

You need to complete both of the measurements for all those headwaters marked with the symbol. You do not have to complete the whole process in one sitting but instead you are able to save and go back into the work at any time.

9. Saving Your Work & Ending MapInfo

Useful tools in this step : Save Table (), Save Workspace ()

First, you will need to save the changes you have made to your layer (table). To do this you can either click on the "Save Table" button on the "Standard" toolbar or go to "File" >> "Save Table...". In the "Save Table" dialog window, click the "Save" button to save your work.

At the very end of the session, you will need to save your work to a MapInfo workspace so that you can retrieve your work in the future. To do that, from the pull-down menu under the "File" command, choose "Save Workspace...". Alternatively, click the "Save Workspace" button on the "Standard" toolbar. Make a note of the location where you save the file. Click "Save" in the "Save Workspace" dialog window and then "Yes" in the popup window to save all the MapInfo files. After that, you can use the "Exit" command from the "File" menu to terminate the application.

Important: To resume work in MapInfo using the newer "lake district.wor" file, skip Step 1 and follow the instructions from Step 2 onwards.

© School of Geography, University of Southampton