Wetland Mapping: Exercise 1 – Create the NDVI Layer ArcMap 10

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 that your 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 let’s 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 Geoprocessing menu and in drop down, click on the Environments… button:

Click on Workspace:

Verify that the Current Workspace is your class OutputData folder and that the Scratch Workspace is set to the ScratchArea folder in your Wetland training folder. Your screen should look like:

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Click OK on this window.

Now click on Geoprocessing > Geoprocessing Options…

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Verify that the Overwrite outputs entry is checked and click OK on the Options window and you’re 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 Data icon in the ArcMap standard tool bar

• Browse to the raster NAIP08.img in 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 Ctrl key, single click both on Band_4 and on Band_3

• Release the Ctrl key and click on the Add button

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 Geoprocessing in the main

menu and click on ArcToolbox to open it as shown on the left.

Open Spatial Analyst Tools > Map Algebra and select Raster Calculator

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Raster Calculator Tool

In the Output Raster window, browse to your Wetland Mapping\ScratchArea folder and enter NDVI in the Name box.

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

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We’re 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 Float math 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 it’s not specified), it could be because you don’t 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, don’t worry about doing the final recode steps described below. In subsequent exercises you will use a pre-made NDVI instead of this all zero raster

Congratulations! You’ve 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 of rock, 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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(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 <= 0.0181

Open the Spatial Analysts tool box and inside the Reclass tool set, double click on the

Reclassify tool:

Either open the Browse icon for the input raster and browse to your NDVI raster created above or simply drag the NDVI raster layer from the ArcMap Table of Contents and drop it into the Input raster entry in the Reclassify tool above. Your screen should look like:

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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 don’t click OK yet:

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 Environments Tab 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 OK to close the Environments

Click OK to run the tool

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The following message may or may not appear. If it does, click Close and the new layer should display (your colors will be randomly set by ArcMap).

This concludes Exercise 1.

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