Embedded Files
This tutorial will cover a few of the raster symbolization options. This tutorial was created using ArcGIS Pro version 3.3.
Before starting this tutorial, you will need to download and install a dataset following these instructions:
Create a folder called Symbolizing_rasters somewhere under your personal directory (e.g. C:\Users\yourname\Documents\ArcGISPro_tutorials\Symbolizing_rasters\).
Download the data for this exercise then extract the contents of Symbolizing_rasters.zip into your newly created Symbolizing_rasters folder.
Tutorial Readings
Tutorial Readings
This tutorial will introduce you to working with raster data representing elevation, land use, and temperature. However, the raster data model is used to represent many different geographic phenomenon, especially those observed using remote sensing. Take some time to review the readings and examples below before you finish this tutorial.
Review these readings as a part of completing this tutorial:
Introduction to Raster Data from Data Carpentry (web page)
Remote Sensing – An Effective Data Source for Urban Monitoring (~7 page paper)
Remote sensing and geospatial technologies in support of a normative land system science (~7 page paper)
Examples of Remote Sensing Applications and Uses: https://gisgeography.com/remote-sensing-applications/
Part 1: Introduction to symbolizing raster features
Part 1: Introduction to symbolizing raster features
Open the map document
Open the map document
The map consists of two raster layers: a continuous elevation raster and a discrete land/water raster.
Manipulating a raster layer's stretched colors
Manipulating a raster layer's stretched colors
When loading a raster, ArcGIS Pro will select a default symbology scheme that matches the raster data type. The elev.tif raster is saved as a float and stores a large range of unique pixel values. As such, the software will default to a stretched symbology using a sequential color scheme (here, it chose a grayscale color scheme).
In this exercise, you will learn how to modify the appearance of a raster layer when exploring a raster.
Enabling the Dynamic Range Adjustment (DRA)
Enabling the Dynamic Range Adjustment (DRA)
When exploring a raster layer at different zoom levels, you might find it useful to have the software make use of the full range of colors in the color scheme at any zoom level so as to distinguish as many different pixels as possible.
Zoom in somewhere on Africa's Western region.
In doing so, the map will appear uniform in color and individual pixels will be difficult to discern. This is because the range in values covered by this extent is small compared to the full range of values in the raster layer. This translates to a small range of usable colors (or shades of gray in our example) in this map extent.
While making sure that the elev.tif raster is still selected, click (enable) the DRA option under the Raster Layer tab. See below if this option is not present in your version of ArcGIS pro.
The newest release of ArcGIS Pro removed the DRA option from Rendering group of the Raster Layer tab. It is also available under the layer symbology.
Navigate to the symbology pane and select DRA under Statistics:
You should now be able to make out some of the individual pixels in the Map's extent.
Changing the stretch type
Changing the stretch type
You can change how the range of pixel values in your raster are mapped to the color scheme. The default is Percent Clip. You can change the stretch type to change the raster's "look". Here are a few examples:
None
Percent Clip
Standard Deviation
With the elev.tif raster selected in the Contents pane, open its symbology pane by clicking on Symbology under the Raster Layer tab.
To see how the raster's values are mapped to the range of colors, you can click on the Histogram icon to the right of the Stretch Type field.
If the software does not allow you to click on the histogram icon, or if you see the following message when hovering the cursor over the icon, you will need to (re)compute the raster statistics.
You have two options:
OPTION 1 (the quickest):
Click in the Classify option in the Primary Symbology pull-down menu. This will bring up a Warning window asking if you want to compute the histogram.
Click Yes.
Reselect the Stretch color scheme.
The histogram button should now be enabled
OPTION 2:
Open the Calculate Statistics geoprocessing tool.
Load the raster dataset. (There may not be a pulldown menu to select the layer from in which case you will need to load the raster via the small folder icon to the right of the field).
Click Run.
Once the statistic is recalculated, you might need to select a different symbology scheme such as Discrete, then switch back to Stretch to enable the histogram button.
For example, if you select the Minimum Maximum stretch type, you will be presented with the following histogram.
The bottom axis displays the range of pixel values in the elev.tif raster and the y axis displays the frequency (number of pixels) associated with each pixel value interval.
You'll notice a diagonal dashed line in the plot. This defines the relationship between the range of available colors and the range of pixel values in the raster. Here, the diagonal dashed line indicates that the pixel values are mapped to each color following a linear relationship. So if the color scheme was made up of 100 unique colors and the raster pixel values ranged from 1 to 200, the raster value interval [1, 2] would be mapped to the first color; value interval [3, 4] would be mapped to the second color; this would continue to the last range of pixel values whereby the raster value interval of [199, 200] would be mapped to the 100th color.
You can change the mapping of the pixel values to the color values by moving the upper and/or lower limits. For example, you can move the upper limit down to around 3700 and the lower limit up to about -6500.
You will now see two histograms. The grey histogram shows the original distribution of pixel values. The red histogram shows the distribution that is actually being mapped to the full range of available colors. By sliding the upper and lower limits, we instructed the software to map all pixel values less than -6500 to a single color (the lowest color value) and all pixel values above 3700 to the maximum available color. This prevents a very small number of pixels in the upper and lower end of the range from hoarding so many individual colors.
You can click on the back arrow to return to the main Symbology window
Creating a custom divergent stretched color scheme
Creating a custom divergent stretched color scheme
You can apply a divergent stretched color scheme to a raster layer. For example, with the elevation raster, we can assign one hue for pixels whose values are less than 0 (water) and another hue for pixels whose values are greater than 0 (land).
Select a divergent color scheme. For example pick the Red-Yellow-Blue option from the Color scheme pull-down menu. This option is near the bottom of color schemes list.
We usually associate blue hues with water. If your reds were assigned to lower values and blues to upper values, you will need to swap the gradient such that reds are associated with higher values and blues with lower values.
If your blues are assigned to positive elevation values, you will inverse the colors (if not, ignore this step).
In the elev.tif layer's symbology pane, click on the Invert checkbox to swap the colors.
Export the map view or take a screenshot that demonstrates you created the land-water color scheme
Divergent color schemes make the most sense when a central value is implied. With the elevation raster, a sensible central value is 0 where negative values (i.e. below mean sea levels) are associated with blue hues and positive value (i.e. above mean sea level) are associated with red hues.
The divergent color scheme would work well if the distribution of elevation values was symmetrical about 0, but it is not. This results in a few red pixels bleeding into the below mean sea level elevation.
The accompanying figure shows the coast of Florida where the land-water boundary is defined by a 0 meter contour line. Note the red hues for elevations below mean sea level.
There are several techniques that can be adopted to resolve this issue. Two such methods are presented next.
Method 1
Method 1
One approach (and the simplest of the two proposed methods) is to adjust the min/max values in either the Percent clip or Minimum-Maximum stretch methods.
Make sure that you are still using a stretch symbology.
Select the Minimum Maximum stretch type.
Enable the Edit min/max values option.
Since the largest range of values about the 0 value is [0, -10182], we will mirror this range on the positive side of the 0 value to ensure a balanced color scheme about the value of 0.
Set the min/max values to -10182 to 10182. Note that if you inverted the color scheme, the max value is the value to the right, and the min values is that to the right.
This works well where the land water elevation gradient is great, but the lighter colors near the central value make it difficult to identify land/water interfaces in areas where the gradient is not as steep (e.g. Florida).
Next we will modify the color scheme by creating an abrupt break in hues as shown here:
Creating a multipart color scheme
Creating a multipart color scheme
Click on the Color scheme pull-down button and select Format color scheme at the bottom of the pull-down menu.
At the bottom of the Color Scheme Editor window, select Multipart Color Scheme.
With Continuous Color Scheme selected from the sub-schemes pull-down menu, click on the Add button twice.
This will add two palettes to the scheme. The default colors shown here may not match those in your project. This is fine given that we will modify the palettes.
Click on the Edit button to the right of the first palette.
Click the left-most color stop on the color scale.
Click on the color pull-down menu and select a dark blue color such as Ultra Blue.
Likewise, click on the right-most color stop of the gradient and change its color to Sodalite Blue.
This color gradient will map the below mean-sea level values. Next, we'll modify the other color gradient.
Click on the back arrow.
Now, click on the Edit button for the second color scheme.
Set the left-most color stop to Topaz Sand.
Set the right-most color stop to Tuscan Red.
Click on the Back arrow to return to the Color Scheme Editor.
If you plan to reuse this color scheme in this project, it may be worthwhile to save it.
Click on Save to a style.
Name it land-water color scheme.
Click OK.
Click OK to return to the symbology pane.
Your new color scheme should now look something like this:
If the Invert box is checked, uncheck it.
The delineation between land and water should now be much clearer now that we have an abrupt break where the two hues converge
Tweaking the color scheme to accommodate a range of values not centered on 0
Tweaking the color scheme to accommodate a range of values not centered on 0
The aforementioned works well when the range of colors cover a symmetrical distribution of values about the central value (the value of 0 in our working example). However, the elev.tif raster has value ranging from -10182 to 6821. We may, therefore, want to assign a wider range of blues to the interval [-10182,0[ than a range of reds for the [0, 6821] interval. In other words, we would might want to go from a symmetrical color palette (left palette in the accompanying figure) to an asymmetrical palette (right palette in the accompanying figure).
Click on the histogram button.
If the histogram button cannot be activated, the statistic needs to be recomputed. See the grey text box earlier in this tutorial for instructions on how to (re)compute raster statistics.
Make sure that stretch type is set to minimum-maximum.
Reset the range, if needed, by clicking on the Reset button.
Click on the back arrow to get back to the main Symbology window.
If you created the multipart color scheme outlined in the previous method, it should be present at the bottom of the color scheme pull-down menu.
If you saved it as a style, you may find the color scheme at the top of the list. If not, follow the instructions in the last subsection to create this color scheme.
With the multipart color scheme selected, click on the Format Color Scheme link at the bottom of the pull-down menu to bring up its Color Scheme Editor window.
You'll notice an arrow at the midpoint of the multipart color bar. This arrow (aka slider) can be moved to the right or to the left to re-center the sub-schemes' cutoff.
The elev.tif raster's values range from -10182 to 6821. The middle value is half way between the min/max values and can be calculated as (-10182 + 6821) / 2 = -1680. If the data were symmetrical about zero, the middle value would be 0. Here, the mid value is to the left of zero suggesting that we will need to move the slider to the right so that the blue hues can accommodate the full range of values between -10182 and 0.
Move the slider towards the red color. Unfortunately, the percentages are rounded to whole numbers. So, we will need to round the percent to 40%. This may not seem like a big deal, but the rounding error will be noticeable along coastal areas having a small elevation gradients such as Florida.
Click OK to accept the change.
Note how the gradient in the legend is shifted to reflect the shift in the multipart color scheme.
Also, note that because of the rounding imprecision in the last step, some below mean sea level pixels will be assigned a red hue.
Take a screenshot of your ArcGIS Pro interface that includes the map extent and Contents Pane (submit on Brightspace)
If you do not see a clear delineation between the water-land pixels and, assuming that you carefully followed the instructions outlined above, you may need to fiddle with the slider bar a few times to get it to hit that 60%/40% marker exactly--even if the widget indicates that the 60%/40% marker is properly set. The following video clip shows an example where toggling back and forth between different values will eventually get you to the desired outcome.
Note that as of version 3.3 of ArcGIS Pro, getting that cut-off value exactly may prove illusive with this working example.
symbology_slider.mp4Categorical color schemes
Categorical color schemes
If the raster layer consists of values that represent categories, you can assign a unique color to each unique value in the raster. In the next example, we will work with the land_water.tif raster layer that consists of just two values: 0 for water and 1 for land.
Turn off the elev.tif raster layer in the Contents pane.
Select the land_water.tif raster layer.
Bring up its symbology pane.
Select Unique Values (this may already be the default).
Note that ArcGIS will usually default to a Unique Values symbology when it identifies a small set of unique, integer values.
Change the colors for each swatches by clicking on them individually. Choose a light shade of blue for water and a dark shade of green/blue for land. In this example, Sugilite Sky is chosen for water and Deflt Blue for land.
Next, change the labels from a numeric value to something more descriptive like Water and Land.
The new labels should now appear in the Contents pane.
Save your work.
Take a screenshot of your ArcGIS Pro interface that includes the map extent and Contents Pane (submit on Brightspace)
Part 2: Use Landsat 8 imagery to calculate and map heat islands
Part 2: Use Landsat 8 imagery to calculate and map heat islands
NOTE: this lab was created in ArcGIS Pro Version 3.2. I've worked through this in the newer versions of the software, but do be in contact if you find yourself looking for a menu/button that is not there.
First, create a new map called "Heat Islands in Albany" in your ArcGIS Pro project.
Heat islands impact some people more than others. As cities face higher temperatures, longer and more intense heat waves, and a stronger heat island effect, many local governments are increasingly focused on protecting the people who are most vulnerable to extreme heat. To accomplish this, many local governments are applying environmental justice principles in their efforts to mitigate heat islands and prepare for extreme heat events.
A growing body of research points to “intra-urban” heat islands, or areas within a city that are hotter than others due to the uneven distribution of heat- absorbing buildings and pavements, and cooler spaces with trees and greenery. These differences can result from disparities in the way communities are planned, developed, and maintained.
--Heat Islands and Equity, Environmental Protection Agency
In this exercise, you will use data collected from a remote sensor, the LANDSAT 8 satellite, to calculate heat islands within the city of Albany, NY.
This method was developed and adapted from researcher and remote sensing expert, Jeff Oppong. Oppong's method uses free LANDSAT data downloaded from the United State Geological Survey (USGS). With this method, you will calculate a Normalized Difference Vegetation Index (NDVI), a Proportional Vegetation Index, Brightness Temperature, Top of Atmosphere (when clicking on this link, see definition supplied by Tomas Brunclick) and correcting for error before creating the final Land Surface Temperature output raster.
Note: we will not spend much time on the calculations, but do be sure to take time to understand what the calculations generate: NDVI, brightness temperature, top of atmosphere, etc. using the links above.
Model of the layers you will create
Download and unzip the provided LANDSAT images for September 13, 2016 (LINK TO DATA) and place in your project folder.
Go to Add Data and navigate to your folder containing the LANDSAT images and add Band 4 and Band 5 to your map (those files ended in B4 and B5). Select Yes when prompted to “Calculate Statistics.”
Click OK to accept defaults in the windows that pop up after you add the layers.
Shorten each name to “B4” and “B5” respectively and save your project.
Go to Analysis > Geoprocessing > Tools and search and open “Raster Calculator” (Image Analysis Tools). Set the parameters as shown in the image below in the expression window.
Change the name of the Output raster to “ndvi”
Note: it is best practice to select the rasters from the Rasters window and the tools and operators [i.e., plus sign, divide, multiply, etc.] from the Tools window by double-clicking on the entry you wish to place in the Map Algebra expression to avoid spelling errors or incorrect entry; you will need to enter the numbers and some of the parenthesis, however.
Float("B5"-"B4")/Float("B5"+"B4")
BEFORE clicking on the run button, however, it is critical to change your analysis environments settings – you can do this from the Raster Calculator tool. Open the Environments tab and set the Output Coordinate System to the raster layer B5.tif [WGS_1984_UTM_Zone_18N] and make sure the Processing Extent is set to Map Extent. Use the map icon to set to map extent.
Return to the Parameters tab and click the Run button. Save your project.
Add Band 10 and rename “B10.” Open the raster calculator and enter the parameters shown in the expression window. Output raster = toa
0.0003342 * "B10"+0.1
Click Run
Open the raster calculator and add the formula as shown in the image.
(1321.0789/Ln((774.8853/"toa")+1))-273.15
Change Output raster to bt
Run
Calculation of Proportion Vegetation Index (PVI) from NDVI
Calculation of Proportion Vegetation Index (PVI) from NDVI
Open the raster calculator and add the formula as shown in the image.
Square(("ndvi"+1)/(1+1))
Name the output pvi
Run
Turn off all layers created in the previous steps to display only the newly created layer on your map
Calculation of Error Correction (EC) from PVI
Calculation of Error Correction (EC) from PVI
Open the raster calculator and key the formula as shown in the image below; run.
0.004*"pvi"+0.986
Name the output e_c
Turn off all layers created in the previous steps to display only the newly created layer on your map
Calculation of Land Surface Temperature (LST)
Calculation of Land Surface Temperature (LST)
Open the raster calculator and key the formula as shown in the image below; run.
("bt"/(1+(0.00115*"bt"/1.4388)*Ln("e_c")))
Change Output raster to lst
Now you should end up with a layer with surface temperature values shown in Celsius degrees.
Turn off all layers created in the previous steps to display only the newly created layer on your map
Calculate the Urban Heat Island (UHI)
Calculate the Urban Heat Island (UHI)
A careful review of the lst dataset you just created shows an error – an “artifact” - was introduced by pixel values on the far east and west edges of the satellite imagery.
You will use the Set Null tool to rectify this problem, which will eliminate this data from your analysis. NOTE: during this step using Set Null you might be thrown an error that tells you there is a problem due to a space in your file path. If you get this error, you will need to use the browse button (folder icon) to locate the lst file within your geodatabase.
Go to Analysis --> Geoprocessing --> Tools and search and open “Set Null” (Image Analysis Tool).
Use the following parameters and run:
Input conditional raster = lst
New expression:
Value is less than 0
You might need to enter the SQL expression: VALUE < 0
Input false raster or constant value: lst
Output raster: uhi
REMINDER: Turn off all layers created in the previous steps to display only the newly created layer on your map
Select your newly created layer and choose symbology. Select “Red-Blue Continuous” to create a meaningful ramp (you may need to check invert to show cooler temperatures in blue and warmer in red) and use the following parameters:
Stretch type = Percent Clip
Min = 0.500
Max = 0.500
Note: applying new symbology may also take several minutes to display, be sure you are at 1:75,000 or below. Click the refresh button in the bottom right corner of the map extent as needed
Change the base map to imagery, locate Albany, NY and zoom to 1:200,000.
Set the uhi layer transparency to approximately 50%.
Create a heat island map layout
Create a heat island map layout
Create a layout for your heat island layer. The map layout should include a brief title, scale bar, north arrow, and brief map description. Remember to use guides to create a clear, well-formatted layout document.
Search the Portal --> Living Atlas for the USA NLCD Impervious Surface Time Series and add to your map.
Toggle your heat island raster layer on and off. What do you see? (include a short response in your Brightspace submission)
Create a layout for your heat island layer. Ensure all layers are turned off except uhi (with transparency set to approximately 50%) and the base map is set to imagery.
Add two vector data indicator layers for comparison
Add two vector data indicator layers for comparison
What additional data would help you better understand the spatial distribution of the indicators you have mapped thus far?
Use the Supplemental Resources provided on the ArcGIS Tutorials site to identify two relevant indicators (e.g., ArcGIS Living Atlas, mySidewalk)
Add the indicators as a feature class in ArcGIS Pro
Appropriately symbolize the layer
Toggle the new layers on and off to compare with your impervious surfaces and heat island layers
What do you see? (include a short response in your Brightspace submission)
Turn off all layers except the imagery base map and the heat island raster. Ensure you have applied a 50% transparency to the heat island raster and ensure the map is displaying to the full extent (i.e., City of Albany at approximately 1:75,000).
Create a two layouts to display the additional indicators, naming them “raster_additional_indicator_name_of_the_indicator”
These maps should clearly display the spatial distribution of your chosen indicators (it should be the most prominent layer).
Each map layout should include a brief title, scale bar, north arrow, and legend. Remember to use guides to create a clear, well-formatted layout document.
Submit on Brightspace
Submit on Brightspace
A word processing document with:
A response to the writing prompt: Name and describe two applications of remotely sensed data. You are encouraged to select two examples related to a research discipline with which you are familiar. (approximately 150-200 words)
Answers to the short prompts throughout the urban heat island tutorial
Screenshots:
Screenshot that demonstrates you created the land-water color scheme
Screenshot of the result of the steps to adjust the color scheme to accommodate a range of values not centered on 0
Screenshot of the categorical raster showing land and water
A heat island map layout (displaying only the UHI layer)
An impervious surfaces layout (displaying only the impervious surfaces layer)
Two layouts to display the additional indicators you selected (each displaying only one additional indicator, one layer)
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