Delineating Lake Drainage Basins in ArcGIS

47 Pages·2015·12.68 MB·English

by  Thomas J. Ballatore

Delineating Lake Drainage Basins in ArcGIS

Prepared by Thomas J. Ballatore, Ph.D.
Director, Lake Basin Action Network (LBAN)
Affiliated Scientist, Center for Ecological Research, Kyoto University
Visiting Researcher, International Lake Environment Committee (ILEC) Foundation
Please send questions and comments to [email protected]

Purpose of this Tutorial

This tutorial provides a step-by-step guide for delineating lake drainage basins using ArcGIS
software. It uses the Lake Biwa drainage basin as an example but the techniques presented
here are fully applicable to the lakes (and other waterbodies such as wetlands, lagoons, rivers,
and so on) you may be currently working on in your own countries.

Checklist of Prerequisites

¨ Completion of “Getting Up and Running with ArcGIS” tutorial.
¨ Installation and activation of ArcGIS (version 10.0~10.2) including Spatial
Analyst extension.
¨ Internet access.

Notes

How to proceed. This tutorial contains a combination of (1) explanations/comments and (2)
steps that you must carry out. You should read it in order from the beginning to the end. To
highlight the specific steps you need to carry out on your computer, I have put those in red
and made them stick out into the left margin a bit. In general, these must be completed in
order (with a few exceptions) so please make sure you have completed each one before
moving on. I have also included Questions at various places. These are optional and designed
to further your understanding.

Data. All of the data used in this tutorial are available free of charge from their respective
sources. Links are given at the appropriate places below.

ArcGIS version. ArcGIS 10.x is quite different from ArcGIS 9.x. For those of you running
ArcGIS 10.0 or 10.1, the differences with 10.2 should be quite minor. However, if you are
running 9.x, please ask me for the older tutorial and I will be happy to provide it. One of the
biggest changes to 10.x has been with the Raster Calculator.
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Preliminaries
Step 1. Deciding what data are necessary, i.e. Where is your basin?

There are many ways you could delineate a given lake’s drainage basin. You could:

1. Use a printed topo map and manually judge from the contours where water is
likely to flow;
2. Use a remotely-sensed image to see rivers and ridgelines to determine which way
water is likely to flow; or
3. Use digital elevation data in GIS.

The last one is the technique we will use and is by far the easiest and most replicable. You
can think of options (1) and (2) like writing an essay by hand and (3) like using a word
processor.

Figuring out the extent you need to cover (the range of data in latitude and longitude) is
tricky because you need to obviously have enough to cover the whole drainage basin but you
don’t want to have too much extra because that slows processing time. In other words, you
need to have a rough idea where your drainage basin is before you actually delineate it with
precision! If you select data that does not cover the full extent, you will have to start over. If
you choose an extent much greater than needed, your processing times as well as file sizes
might become prohibitively large.

You might already have drainage basin maps that give you a good idea, or you might simply
have first-hand knowledge that allows you to make a sensible guess about extent. One
technique that I often use when working with drainage basins I am not very familiar with is
to:

Step 1.1. Open Google Earth (available for free at http://www.google.com/earth/index.html)

Step 1.2. Make sure the Latitude/Longitude grid is turned on (usually Cntl+L)

Step 1.3. Zoom to the area of interest and look at the topography and rivers and make an
estimate of the North-South-East-West extents that would enclose the basin
comfortably.

Step 1.4. Optional. You might want to use the Add Polygon or Add Path tool to leave a
record of your tracing. An example for the Lake Biwa basin is given below.

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I made that by just looking at mountains and rivers and roughly estimating the limits of the
drainage basin. Note that I have zoomed out so you can see the lat/long grid better but
naturally I was zoomed in when tracing it the polygon.

In terms of latitude and longitude, the basin is probably completely within by a polygon from
E135~E137 and N34~N36.

Challenge Question: What other methods besides Google Earth might be useful in
determining the necessary extent?

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Working with Features (Vectors)

There are two major data types that are used in a GIS: features and rasters. Features,
sometimes called vectors outside of the ArcGIS world, are like the fonts on your computer
screen: no matter how much you zoom in, they are always “crisp”. This is because they are
drawn based on mathematical rule governing their shape. On the other hand, rasters are like a
photograph: they are made up of individual pixels. If you zoom in, you will see a “jagged”
look. Their resolution depends on your camera and on the way you might have processed the
image.

They have complementary strengths and are both used in modern GIS analysis. It is possible
to convert one to the other and back, and sometimes we will do that, but in general we let
features be features and rasters be rasters.

Step 2. Downloading Waterbody Features

Before we start working with the elevation data, it is nice to have a view of the waterbody
locations in our target area. One of the best global-scale data sets for waterbodies is NASA’s
Shuttle Radar Topography Mission (SRTM) Water Body Dataset, known as SWBD, which
shows the location of water bodies during a February 2000 Space Shuttle mission that were
greater than approximately 600m in height and 183m in width.

You can download this data from:

http://dds.cr.usgs.gov/srtm/version2_1/SWBD/

The files we want are in the SWBD_east folder. The files contain data on 1x1 degree tiles and
area named based on the coordinates of their lower left corner (southwest). For the Lake
Biwa basin, we need the following 4 tiles:

e135n34e.zip
e135n35e.zip
e136n34e.zip
e136n35e.zip

These correspond to the four tiles we identified in the Google Earth exercise above. Note that
the “e” before .zip represents “Eurasia” for these tiles. For other regions of the world, the
letter used is different (e.g. “s” for South America”).

Step 2.1. Make a new folder called C:\GIS\Data\SWBD

Step 2.2. Download the 4 files and save them there.

Step 2.3. Then, make a new folder called C:\GIS\Basins\Biwa\SWBD and unzip the
downloaded files there. You should end up with something that looks like the
following depending on your Windows version and display preferences:

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Notice that each file unzips to 3 distinct files: .dbf file, .shp file and .shx file. This set forms
what is called a shapefile (a common type of vector file) and illustrates one of the difficult
parts of working with GIS as compared with other programs such as Microsoft Excel, for
example. Let’s say you want to share the e135n34e shapefile with a colleague. If you copy
only e135n34e.shp and forget e135n34e.dbx and e135n34e.shx, the file will not be readable.
You need to be very careful about this while using Windows Explorer. A better option is to
use an application called ArcCatalog (part of ArcGIS) when copying, moving, or transferring
files.

Challenge Question: What other ways besides SWBD could you get information about your
waterbodies of interest? What if your waterbody has variable size, location, etc. through
time?

Step 3. Defining a Projection

The SWBD shapefiles you just downloaded are not yet ready for use in ArcGIS because they
do not have a “projection” defined. We know from the NASA metadata that the files are in
Geographic Coordinate System (GCS) using the datum WGS_1984. To ensure that the data
appears in the correct location, we must define the projection with a tool in ArcMap.

Step 3.1. Open the map Biwa.mxd you saved as part of the “Getting Started with ArcGIS”
Tutorial. (Note: If you don’t have this, just start ArcMap, open a new map, then
save that as Biwa.mxd in C:\GIS\Basins\Biwa)

Step 3.2. Go to the ArcToolbox and double click on the Define Projection tool located at:
Data Management Tools | Projections and Transformations | Define Projection

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Double clicking the Define Projection tool in the
ArcToolbox (to the left) brings up the Define
Projection tool dialog (shown below).

Step 3.3. In the dialog, click the icon and navigate to C:\GIS\Basins\Biwa\SWBD and
add e135n34e.shp as the Input Dataset

Step 3.4. Next, click on to select the Coordinate System. You will need to navigate to
WGS 1984 inside the Geographic Coordinate Systems folder. Select this as shown
below and click OK.

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Step 3.5. Click OK in the Define Projection dialog as shown below to execute the tool.

The tool will execute and, if successful, three things will happen. First, you will see a dialog
indicating completion:

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Second, three new files will be written in C:\GIS\Basins\Biwa\SWBD, namely e135n34e.prj
along with an .xml file and a .lock file (this one will disappear when you close ArcMap).

Note that if you open the .prj file with Notepad, you can see the contents:

This is simply the information that we selected when we chose the projection as
GCS_WGS_84.

Third, the shapefile will be automatically added to ArcMap. You should see something that
looks like this:
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Note that the color assigned to the polygons is random and the one you see might be different.

Step 3.6. Repeat Step 3.5 for the remaining 3 shapefiles.

Step 3.7. Click the “Full Extent” button in the Tools toolbar, you should see something
like this:

Lake Biwa is clearly visible, as are Ise Bay (southeast), Osaka Bay (southwest) and the Sea of
Japan (northwest).
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Step 4. Merging Features

Right now, we have 4 separate shapefiles being displayed as separate layers (on the left-hand
side of the ArcMap window). Lake Biwa itself is divided in pieces among three of them.
Ideally, we’d like to have each waterbody as an independent “record” within a single, all-
encompassing shapefile. This is a several-step process. First we need to merge the four
shapefiles into one as follows:

Step 4.1. In ArcToolbox, go to Data Management Tools | General | Merge and open the tool.

Step 4.2. For Input Dataset, click the icon and navigate to C:\GIS\Basins\Biwa\SWBD.
Add all four shapefiles.

Step 4.3. For Output Dataset, click on the next icon, navigate to
C:\GIS\Basins\Biwa\SWBD and enter SWBD_merge.shp.

Step 4.4. If everything looks like the figure below, click OK. (Note that if you have a very
large number of files to process, you can right click on the Merge tool in the
ToolBox and use the “Batch” option.)

A new shapefile called SWBD_final.shp will be automatically added to ArcMap. Note that
the color of all the polygons shown will be the same. This indicates that each one is an
individual record in the same shapefile.

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