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Wetland Mapping: Exercise 1 Create the NDVI Layer

ArcMap 10

Note: In the following instructions you will be asked to browse the directory location of your Wetland Mapping training folder. That location is

represented by a drive letter of x: Please note that the screen shots used in the examples were created utilizing data that was located on

C:\temp (i.e. the value of x: was C:\temp). Be sure to use the actual drive letter assigned by the computer to your flash drive and be aware thatyour screens will reflect that letter and not C:\temp as in the samples. Check with your instructor for more details if you have questions.

Open ArcMap 10

First lets make sure that your default workspaces are available to you for all of these exercises.

If this step is not done, you may get errors later on because ArcGIS cannot access workspaces

properly. In the Arc Map 10 main menu bar, click on the Geoprocessingmenu and in drop

down, click on the Environmentsbutton:

Click on Workspace:

Verify that the Current Workspace is your class OutputDatafolder and that the Scratch

Workspace is set to the ScratchAreafolder in your Wetland training folder. Your screen should

look like:

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Click OKon this window.

Now click on Geoprocessing > Geoprocessing Options

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Verify that the Overwrite outputs entry is checked and click OKon the Options window and

youre set to go for the rest of the session.

From the main menu bar, click on the File menu and click on Open

In the Open window, browse to x:\WetlandMapping\Exercise1.mxd(where "x" is the

letter of your usb drive) and click on that icon to open the ArcMap document for this exercise.

Your screen should look something like:

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You should see a false color infrared image of the Chanhassen study area. Notice in the ArcMap

Table of Contents that Band 4 (NIR) is assigned to the Red color, Band 3 (Red) is assigned to the

Green display color and Band 2 (Green) is assigned to the Blue display color.

In order to create an NDVI layer, we must load the Red (Layer 3) and Near InfraRed (Layer 4)

image layers separately into the ArcMap Table of Contents. We will start with the NIR Layer 4:

Click on the Add Dataicon in the ArcMap standard tool bar

Browse to the raster NAIP08.imgin ClassData folder of your WetlandMapping folder

and double click on the icon; You should see the following in the Add Data window:

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While holding the Ctrlkey, single click both on Band_4 and on Band_3

Release the Ctrlkey and click on the Addbutton

Your screen should now look something like this:

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Take some time to explore the data. If you unclick all of the layers in the Table of Contents

except Band_4, you will be seeing a black and white representation of the intensity of the

image in Near InfraRed. Notice how the lakes and waters are essentially black in NIR while they

show shades of gray in the Red (3) layer. We can now use these two new layers to do some

image algebra to create our NDVI layer.

We will use a tool in the Spatial Analyst Tool Box to create our NDVI layer. If

the ArcToolbox window is not visible, click on Geoprocessingin the main

menu and click on ArcToolboxto open it as shown on the left.

Open Spatial Analyst Tools> Map Algebra

and select Raster Calculator

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Raster CalculatorTool

In the Output Raster window, browse to your Wetland Mapping\ScratchArea folder and enter

NDVIin the Namebox.

Click on the Savebutton of the Output raster window. Your screen should now look like:

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Were now ready to build the raster calculator equation that will create our NDVI layer. You

have two options to build the equation. Cut and paste or build it step by step. Choose either

way below.

Build the expression the easy way

Copy and paste the following expression into the Map Algebra expression box:

Float("NAIP08.img - Band_4" - "NAIP08.img - Band_3") / Float("NAIP08.img - Band_4" + "NAIP08.img - Band_3")

Or, if you'd like to experiment, you can build your own expression step by step:

Click in the open box for the expression building

In the Math function list, double click on Float. The word float should appear in your

expression window with opening and closing parentheses.

Browse to the NAIP08.img -Band_4 and double click on it. The name of this layer should

appear in the expression box.

Click on the Minus (-) button in the calculator operators

Then double click on the NAIP08 - Band_3 entry in Layers and variables

Click at the end of the expression once and then click on the / button

Double click on the Floatmath operator

Double click on NAIP08.img-Band_4 in the Layers and variables section

Click on the + button

Double click on the NAIP08.img- Band_3 in the Layers and variables section

Both techniques of entering the expression should give you a screen that looks like:

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Click OK. The tool should run for a few minutes (and may show a status bar of 100% for many

of those minutes- it is ArcGIS after all!) Depending on the site location you may or may not get

the following window. If you do, click Close upon completion.

If the tool runs for a few seconds and says there was an error (but its not

specified), it could be because you dont have write access to the work areas on

the computer. Rerun the tool and click on the Environment button on the

bottom of the tool dialog box. Under General Settings, specify both a Current

Workspace and Scratch Workspace that is in your C:\Temp directory

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After completion of the process, Arc Map will load your new layer to your map

Special Note: There is a reported bug in ArcMap 10 with some operating

systems that trying to divide integer rasters (like we are doing here) results in an

output raster that is all zeros. If this happens to you, dont worry about doingthe final recode steps described below. In subsequent exercises you will use a

pre-made NDVI instead of this all zero raster

Congratulations! Youve created your NDVI layer. This excerpt from the ESRI NDVI web page

(http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Using_the_NDVI_process)

talks a little bit more about the NDVI. This produces a single-band dataset, mostly representing

greenness, where any negative values are mainly generated from clouds, water, and snow, and

values near zero are mainly generated from rock and bare soil. This index outputs values

between -1.0 and 1.0. Very low values of NDVI (0.1 and below) correspond to barren areas ofrock, sand, or snow. Moderate values represent shrub and grassland (0.2 to 0.3), while high

values indicate temperate and tropical rainforests (0.6 to 0.8) (Ref:

http://earthobservatory.nasa.gov/Features/MeasuringVegetation).

We now have one final step left. Since NDVI represents essentially greenness we want to

recode our output to be a raster that will have a value of 1 where we would expect wetlands

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http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Using_the_NDVI_processhttp://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Using_the_NDVI_processhttp://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Using_the_NDVI_processhttp://earthobservatory.nasa.gov/Features/MeasuringVegetation)http://earthobservatory.nasa.gov/Features/MeasuringVegetation)http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?TopicName=Using_the_NDVI_process

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(higher NDVI) and a value of 0 where we would not. Our research indicated that an NDVI value

of 0.0181 resulted in a reasonable threshold. In the next step, we will use ArcGIS to recode the

raster as 0 anywhere the NDVI

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Select all of the table entries except the first and the last two by clicking in the first column of

the table on line 2 and dragging down to the bottom (the red arrow above shows you where to

click to select a row):

Click on Delete Entries. You screen should look something like:

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Change the ranges in the old values and the new values in the table so they look like and enter

an output file name of NDVI_R in your output folder (see figure below) but dont clickOKyet:

Please make sure to check the box next to Change missing values to NoData:

Also, we want to make the output cell size be 3 meters to match our other data later, so

click on the EnvironmentsTab at the bottom of the window and scroll down to the Raster

Analysis section. Change cell size to be As Specified Below and enter 3 into the box:

Click OKto close the Environments

Click OKto run the tool

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The following message may or may not appear. If it does, click Closeand the new layer should

display (your colors will be randomly set by ArcMap).

This concludes Exercise 1.