The document is an open training module for geographic information science focused on watershed delineation, specifically developed for students at UBC. It includes instructions on utilizing SRTM data and ArcMap software to derive streams, basins, and analyze watersheds, particularly in the context of studying the Faro River basin's potential to cross the Cameroon-Nigeria border. The training aims to equip learners with practical skills for creating digital elevation models and responding to geographic questions.

1. Watershed
Delineation
March 2016
Dr. Arthur ‘Gill’ Green
arthur.green@mail.mcgill.ca

2. What is This?
Watershed Delineation by Arthur Gill Green is licensed under a Creative Commons Attribution-
ShareAlike 4.0 International License.
Based on a work at http://greengeographer.com/.
• An open training module for learning
Geographic Information Science as applied to
watershed delineation.
• It was developed for students at UBC.
• It uses data from the USGS (SRTM) and
software from ESRI (ArcMap 10.3).

3. Why Do This Training?
It’s free and you will learn how to:
• Get SRTM 1 Arc-Second (30 meter resolution) for
free to make Digital Elevation Models (DEM).
• Merge raster grids into mosaics.
• Derive streams, stream orders, basins, and specific
watersheds from the data.
• Convert raster grids into vector features.
• Calculate area and length.
• Create and analyze evidence for responding to
geographic questions.

4. Research Question
While working in Cameroon on a transboundary
international conservation area near Tchabal
Mbabo, we wondered…

5. Does the Faro River basin cross the international
border between Cameroon and Nigeria?
If it does, this is one reason to explore setting up a
transboundary international conservation area or
international watershed co-management plan.

6. Where are Tchabal Mbabo &
the Faro River?
• This photo is from the Tchabal Mbabo cliffs
looking down into the Faro River basin.
• This is a remote region, located in the west of the
Adamaoua Province of Cameroon. Livelihoods
are based around herding.
• The rapid drop of the cliffs provide many
microclimates and are home to rare Afromontane
and Sudano-Guinean flora and fauna. For
example, Prunus africana is one rare species
here.
Click below to see
the region on OSM.

8. What You Need
• Software: ArcMap 10.3.
• A license to use Spatial Analyst.
• Access to the internet.
• 3-4 hours (depending on the size of the data
you download and your computer’s processing
abilities it could be even longer – so choose a
small area).

9. Outline
1. Get Data
2. Set Work Environment
3. Mosaic Rasters (put them together)
4. Find Sinks and Fill Sinks (create a depressionless DEM)
5. Flow Direction
6. Flow Accumulation
7. Basins and Watersheds
8. Stream Network
9. Converting to Polygons and Cleaning Up
10. Comparing Basin to Watersheds
11. Answering the Research Question

10. 1. Get Data

11. Get Data
• We will use SRTM (Shuttle Radar Topographic
Mission) 1 Arc-Second Global elevation data
(~30 meter resolution) from the USGS and
NASA. Collected Feb. 2000.
• Get an account at:
http://earthexplorer.usgs.gov/
• I use data from the border of Cameroon and
Nigeria to look at where the Faro River is
located.
https://en.wikipedia.org/wiki/Faro_River
• You can select your own region. Make sure
to only take 1-2 SRTM image areas or your
computer may take a very long time to
process the data.
Source:
http://www2.jpl.nasa.gov/srtm/mis
sion.htm

12. • This is the http://earthexplorer.usgs.gov/ interface.
• Sign up for a free account.
• Once you have a free account, you can search for data
from any part of the world or manually outline your area of
interest (AOI).

13. • I manually outlined my area of interest using the above
points.
• Once you have outlined your area of interest, you can
move on to searching the types of data available. We are
going to search for SRTM.

14. • After outlining your area
of interest (AOI), move
on to the Data Sets tab.
• Type in “srtm” and you
should see SRTM 1 Arc-
Second Global.
• Select that, then click on
Results.

15. • You can show the SRTM image footprints by using the buttons on the
left.
• We see there are two SRTM images that fall within my AOI.
• I will have to download both of them and mosaic (merge) them into a
new raster.
• You can display metadata (SRTM files were collected in 2000 but
published in 2014). You can do individual or bulk downloads.

16. Get Metadata

17. Product Specifications (some metadata)
Projection Geographic
Horizontal Datum WGS84
Vertical Datum EGM96 (Earth Gravitational Model
1996) ellipsoid
Vertical Units Meters
Spatial Resolution 1 arc-second for global coverage
(~30 meters)
3 arc-seconds for global coverage
(~90 meters)
Raster Size 1 degree tiles
C-band Wavelength 5.6 cm
Projection is geographic, so we will need to reproject to do
measurements (area, length) in meters.

18. Download Data

19. Download Data
• Earth Explorer offers SRTM data as:
• Digital Terrain Elevation Data (DTED)
• Band interleaved by line (BIL) (a binary raster format)
• Georeferenced Tagged Image File Format (tif, tiff,
GeoTIFF)
• Any of these formats will work for this exercise.
• I downloaded the GeoTIFF.
• You should make a project directory (such as
“C:/WATERSHED/DATA”) and move/unzip the files
into that directory.
https://lta.cr.usgs.gov/SRTM1Arc

20. 2. Set Work
Environment

21. Map Document Settings
• Create a new mxd document in the same folder as
your data. Call it “watershed.mxd”
• Enable Spatial Analyst tools Customize > Extensions >
Spatial Analyst. Search the Hydrology toolbox.
• Set Map Document Properties

22. Geoprocessing Settings
• Create a new file geodatabase in your DATA directory using
ArcCatalogue. You can name it what you want.
• Go to Geoprocessing > Environment and enter the following
settings.

23. Load Data and Check
Properties
• Load your geodatabase and images into the
mxd.
• Check your data properties.
• I have 1 band and 16 Bit pixel depth. None of
the data values are negative so, I can use a 16
Bit Unsigned when I mosaic rasters in the next
step.

24. 3. Mosaic Rasters

25. Mosaic Rasters
We need to
combine images to
have one raster =
Mosaic To New
Raster.
The new raster
name is
“mosaicsrtm1.tif”.

26. Mosaic Output =
mosaicsrtm1.tif

27. Enhance Visualization: Histogram
Stretch & Color Scale

28. Nigeria/Cameroon border

29. 4. Find Sinks &
Fill Sinks
(Creating a depressionless DEM)

30. Find Sinks
• Sinks are a common problem in DEM. “A sink is a cell or set of
spatially connected cells whose flow direction cannot be assigned
one of the eight valid values in a flow direction raster.”
• Sinks are often data aberrations and they will impact and possibly
ruin models of flow direction…. Yet, to find sinks we need to first
do a flow direction analysis which assigns cell values that reflect
flow direction.
Flow Direction Results, Source:
http://desktop.arcgis.com/en/arcmap/latest/tools/spatial-analyst-
toolbox/how-flow-direction-works.htm
Up to 4.7% of the cells in a 30 meter DEM
might be sinks.
Tarboton, D. G., R. L. Bras, and I. Rodriguez–
Iturbe. 1991. "On the Extraction of Channel
Networks from Digital Elevation
Data." Hydrological Processes 5: 81–100.
http://dx.doi.org/10.1002/hyp.3360050107

31. Flow Direction to Find Sinks
Sinks are assigned a value of the sum of
their possible directions. You can see
below that the results do not give the
actual flow direction grid that we want,
they give us a raster with values 1-255.
We can use this to identify sinks.

32. I zoomed into a small AOI. These
dots are all sinks and peaks in the
data that we need to fix. Notice how
an apparent stream bed has a
number of low and high values.

33. Fill
• We need to run Fill on the original mosaic
raster. Then we will run the Flow Direction
again.
• Fill will create a depressionless DEM.

34. 5. Flow Direction

35. Flow Direction
• Now we can do a flow direction analysis on a
depressionless DEM. (Notice the difference in our results
now versus using the uncorrected data previously!)
• We can include a drop raster that models drops in elevation
too.

36. AOI Flow Direction

37. AOI Drop Raster

38. 6. Flow Accumulation

39. Flow Accumulation
• Now that we know flow direction we can do a
number of additional analyses.
• Flow Accumulation counts the number of cells
that flow into a particular cell.

40. Flow Accumulation
• Deriving streams from flow accumulation requires examining your
data and that you make a threshold decision.
• Here I have made two classes of the flow accumulation grid, as a
result any cells with more than 5000 cells flowing into them will be
recognized as part of the stream network.

41. AOI Flow Accumulation Classification

42. AOI Flow Accumulation Classification
over satellite images (visual check).

43. 6. Basins and
Watersheds

44. Basins and Watersheds
• There is a function (Basin) that will
automatically calculate the basins in your data
set using flow direction.
• There is a way for us to choose to model
specific watersheds by establishing pour points
and looking at flow accumulation.
• Let’s look at how to do both of these and then
compare our results. Watersheds will take
longer, so let’s start by making basins.

45. Results of the
Basin tool.

46. Watersheds
• Pour points are outlets of the watershed that
you are interested in mapping.
• You can upload a predetermined set of pour
points (based on known locations) or you can
establish your own set of points.
• You will first need to create an empty shapefile
(or geodatabase feature class) for your points.
You can do this in ArcCatalogue. Call the new
layer “pourpoints”.

47. Watersheds
• When creating the new
feature class, create a
field called UNIQUEID
using Short Integer Data
Type.
• This will be used to
identify watersheds.

48. Pour Points
• Load the new layer
you created.
• Start editing the file.
• You may need to
open the Create
Features window to
make the points.

49. • Add points to
your new feature
class as close as
possible to the
stream network.
• Assign an
“uniqueid”
number to each
point.
• Save your edits
and stop editing
when done.

50. • Snap pour points to the highest
point of flow accumulation near
them.
• Snap to Pour Point will do this
and it will convert the points to a
new raster grid.
• Try several snap distances to
avoid having all points go the
same location (snap distance to
large) or miss the stream network
completely (snap distance too
small).
• We have a geographic coordinate
system so the distance is
measured in decimal degrees (1
decimal degree is ~111 km at the
equator but changes further from
the equator).

51. • Snap pour point distance has big
impacts on watershed generation.
• Distance 5 (~555km) led all my
points to be collapsed to one point.
• Distance 0 (no movement) led me
to have one good watershed and
one poor watershed.
• Distance 0.005 (roughly 555
meters) allowed me snap my points
to the highest flow cells and keep
three watersheds.

52. Original vector pour points (yellow) shown next to the
new raster pour points snapped to the highest flow
accumulation point within roughly 555 meters.

53. Generate Watershed
Using the three different snap pour
points rasters to generate
watersheds, I found that the 555
meters snap measurement gave the
best results.

54. Erroneous watershed generated
when Snap Distance = 0 km
Only one
watershed
generated
when Snap
Distance =
555km
Three watersheds
generated when
snap distance =
555 meters

55. 7. Stream Network

56. Creating a Stream Network
• In order to get our raster streams into a vector
format and to perform some other analysis, we
need to make a raster that only shows our
streams.
• We will use Raster Calculator (located in the
toolbox Spatial Analyst > Map Algebra).
• Use the formula on the following slide to
generate a new raster with only the streams
represented.

57. This creates a new raster “streams” wherein all cells that
had the flow accumulation value greater than 5000 will
be given the value “1” and all other cells no value.

58. Creating a Stream Network
Linear raster
stream network.

59. • We can perform Stream Link (to assign unique
values to branches of the stream network).
• Also, look at stream order using Shreve or
Strahler approaches.

60. • Shreve adds cumulatively
saying that 1+1=2, 2+3=5,
and 2+2=4.
• Strahler says that when
order 1+1=2 and that 2+3 =
3 and that 2+2=3.
• This makes a big difference
in the number of orders!

61. We can now convert these ordered steams to a vector
(polyline) feature using Stream to Feature.
This gives us a feature
class that includes
attributes for the nodes
and the order (“grid code”)
as well as the length (but
in decimal degrees, so we
need to fix that).

62. Projections and
Calculating Length
• We need to project our data into a projected
coordinate system in order to accurately
measure length and areas.
• There are two ways to do this calculation:
• Project our data into a new feature class in a
geodatabase (automatically will calculate length
and are in meters).
• Project our data into a shapefile, add data fields,
and calculate geometry for the new fields.

63. Projections
• First find a projected coordinate system that is
appropriate.
• For my data I used UTM Zone 33N (which
covers the majority of my region).
• I add the EPSG number to the file name to
identify the projection. The EPSG for UTM
Zone 33N is 32633.
• You can reproject direcly to a feature class
(gdb) or a shapefile.

64. You can also project directly to a
feature class in your geodatabase.
Or use Feature Class to Feature
Class to send the shapefile to your
geodatabase.
Project to a shapefile:
No geographic transformation is
needed as I am using the same
datum.

65. Calculating Length
(geodatabase)
• This is done for you in the geodatabase. See
Shape_Length now in meters.

66. Calculating Length (shapefile)
Add a new field, right click on the field and choose Calculate Geometry,
Now you should see the calculation in your attribute table.

67. You can now change the Symbology (try line width
and/or colors) on your new vector stream network
to visually represent the stream orders, section
length, or other attributes.

68. 9. Converting to
Polygons and Cleaning
Up

69. Basin to Polygon
• Open up the Basin raster attribute table and
sort by count.
• By selecting the row you will be able to visually
identify and select the basin near your
watershed.

70. Basin to Polygon
• The only function you need now is Raster to
Polygon.
• If you don’t select parts of the raster, then you
can get all the basin polygons using the above
function.

71. Basin to Polygon
Now you should have your new basin polygon.

72. Watersheds to Polygon
• Like Basins, we use Raster to Polygon.
• Yet, something funny happens… Open the
attribute table to see.

73. Watersheds to Polygon
• We now have five watersheds because in
converting the raster to a polygon, some cells were
cut off and formed new features.
• Select one of the areas with small or no
Shape_Area (again this is in decimal degree
because we have not projected our data yet).
• Zoom to the selected area (this is an option in the
menu).

74. Watersheds to Polygon
Here we see the original
raster watershed below the
vector watershed.
We have two options, delete
the hanging area or merge
the features.
Given the small size, I simply
deleted the small polygon
features in the table (you may
need to turn on editing).
I then projected (EPSG
32633) the feature class in
the geodatabase. I now have
my three watersheds and the
measurement of area in
meters.

75. 10. Comparing Basin to
Watersheds

76. We now have watersheds.

77. We now have a basin and
rivers.

78. We can compare our basin to
our watersheds and rivers.

79. We can compare our basin to
our watersheds and rivers.
• When we decided to locate pour points, we ended
up not capturing the entire basin and even
including part of another basin.
• This could impact field decisions.
• For example, if we were collecting flow information
with monitors located in the field we might decide
to change the location of our pour points
(monitors) to more accurately represent the entire
basin and maybe even capture 1-2 more basins
with the same amount of monitors.

80. 11. Answering the
Research Question

81. Does the basin or any small
watershed cross international
borders?

82. Does the basin or any small
watershed cross international
borders?
• Yes, the basin appears to cross the border, it is largely in
Cameroon with small parts of it in Nigeria.
• As well there were other basins that, appeared to cross the
border (largely in Nigeria with small parts in Cameroon).
• We could continue on with this analysis identifying and
quantifying the overlap of basins throughout this region.
• Given our findings, we might suggest that the basin and
watersheds overlapping the border are reasons to explore
an international conservation area or an international
agreement on watershed management.

83. Looking Back
You now know how to:
• Get SRTM 1 Arc-Second (30 meter resolution) for free.
• Merge raster grids into mosaics.
• Derive streams, stream orders, basins, and specific
watersheds from the data.
• Convert raster grids into vector features.
• Calculate area and length.
• Create and analyze evidence for responding to geographic
questions.
More information about this region can be found from organizations
such as BirdLife International:
http://www.birdlife.org/datazone/sitefactsheet.php?id=6112

84. End
Did you find an error?
Could something be more clearly explained?
Did you adopt and adapt these materials?
Let me know at arthur.green@mail.mcgill.ca
Watershed Delineation by Arthur Gill Green is licensed under a Creative Commons Attribution-
ShareAlike 4.0 International License.
Based on a work at http://greengeographer.com/.