Analyzing Craters on Mars

The Basics:

Analyze area, slope, and depth of two Mars craters.

Submit your answers to the questions using the answer form found on Canvas.

Data:

The Mars elevation data for this lab was created as a part of the High Resolution Imaging Science Experiment (HiRISE) project at the University of Arizona’s Lunar and Planetary Laboratory: uahirise.org.

The Elevation Model you will be working with is a Digital Terrain Model (DTM). Information about DTM construction and the HiRISE project can be found here: http://www.uahirise.org/dtm/about.phpLinks to an external site.

• DTM: DTEEC.img
• Shapefile: Craters.shp

Note: you will be generating new data with all tools run in this lab.

Prepare the Data

Start a new ArcGIS Pro map project, and add your data.

First and foremost, did ArcGIS provide a basemap for you automatically?
You know enough about spatial reference systems to question and understand what's going on.

1. Inspect the DTM (terrain model). Dig into the data properties, familiarize yourself, be curious, and answer these questions:

 

Creating a hillshade surface

1. Search for and open the Hillshade (Spatial Analyst) tool
2. Input the Mars DTM
3. Use appropriate Naming Conventions and save the output data to your Output folder. (you want to keep the root file name, and what is unique about this layer. This is a hillshade created from the DTEEC layer. Call it DTEEC_HS)
4. Symbolize by setting the DTM above the hillshade in the ToC at 50% transparency

Inspect the results.

What do you expect the range of values to be?
Will these values go into a legend?

 

Slope Surface

 

The slope value is the steepest slope defined by the cell and its eight surrounding neighbor cells.

Slope can be calculated in

• Degrees (0-90), or
• Percent slope (rise over run, multiplied by 100)
  
  

1. Search for and open the Slope (Spatial Analyst) tool
2. Set the Input Raster as the DTM
   1. Slope is calculated from the elevations
3. Follow output naming convention DTEEC_sl_d (slope in degrees)
4. Verify your output location
5. Select units (Degrees)
6. Calculate using Planar method

See tool help for descriptions of units and note that you don’t need to enter a specific Z factor.

Again, slope is calculated from the elevations in the digital terrain model

 

 

The Craters Polygon Shapefile

Open the attribute table and verify that there are two records, one for each crater.

Is there a field containing a unique identifier for each crater?

You are expected to be able to keep track of which crater is which.

For your submission you will be asked for the area, mean slope, and depth of the northern and southern craters.

 

Slope and Elevation Analysis

Calculate how deep each crater is 

Calculate the average slope for each crater

 

We are interested in calculating

• the depth of each crater and
• the mean slope of each crater

Imagine we have two maps: a polygon defining an area and a raster surface with a range of values.
We can use the polygon area (zone) to limit the processing area of the raster in order to summarize the raster values within the polygon area. Summarizing can include calculating totals, ranges, averages, etc.

We can use the polygon boundaries of the craters to define a space. Only the raster cells that fall within that space will be summarized. The summary can tell us:

• The lowest raster cell value (minimum)
• The highest raster cell value (maximum)
• The difference between lowest and highest values (range)
• The average of all the cell values (mean)
• The standard deviation of that mean value
• The most common value (majority)
• The least common value (minority)
• And even more (like how many cells there are within the polygon area)

This is called “Zonal Statistics”.
Arc calculates summary statistics for user defined ‘zones’.

Pretty great, right? We can run a tool that will look at the cells of the slope raster that are inside each of the crater polygons and it will calculate the mean slope value for us. Voila.

Likewise, we can run it a second time and tell the tool to summarize the elevation raster (the DTM) and it will tell us the highest and lowest elevation values and even difference them for us, telling us the depth of each crater.

 

Zonal Statistics as a Table

This is a tool that:

• looks at a particular zone (or region) and
• analyzes values from a raster, then
• produces summary statistics (min, mean, max, range, etc.)
• and outputs the results to a standalone table

Search for and open Zonal Statistics as a Table (you can actually just search for “zonal” and it’ll come up in the list, to save time…)

• Use your noggin, the explanation above, and the tool help, to set up the tool.
• Run it twice, once on the Slope value raster, and once on the DTM (elevations).
• Don’t forget! You have to put in a field with a unique identifier to get summary statistics for each crater.
• Analyze the results (by looking at each output table and reading them to know they are right and that they make sense.

How can you know if the results are right?

Example: Slope.

1. Are there two records in the zonal statistics table? Yes, good start. No? Run it again and reread the last two pages.
2. What are the min and max values? Do they look like the min and max slope values in degrees (0-90)? If the units are degrees and you have values of thousands, something is wrong, run it again.

Example: Elevation

1. There must be two records in the table or something is wrong.
2. Again, look at the min and max values. Do they agree generally with the range of values seen in the table of contents for the elevations?

Note: the results are just a table. No associated or corresponding spatial data. These results cannot -on their own- be displayed on the map.

 

 

Calculate area

 

This is a chance to practice a very common task.
Calculate the area of both craters. (No Hints: You know how to, and can, do this!)

Calculate area geodesically in units of hectares.

 

Symbolizing rasters

 

Color Schemes

 

 

How you show colors is important as well. There are several ways that color schemes are set up.

Reminder:

• Sequential (continuous):

Sequential color schemes show an obvious gradation from light to dark or one color to another with a clear trend.
Sequential colors are used to represent ranked values. The idea is to increase the intensity of a color as the values increase.

This is a continuous scheme because there is a smooth transition from light to dark. 

Sequential data is used for ranked numeric data, whether continuous values or ordinal (good-better-best) data.

 

• Sequential (discrete):
  

Sequential discrete also represents ranked data, but the data are organized into classes or groups.

 

• Divergent

Divergent color schemes can be continuous or discrete/classified. There is a neutral color in the middle that denotes a change in value or meaning, and the values increase or decrease from a meaningful center point (no change, national average, a meaningful threshold).

 

• Categorical

Nomical data is displayed using a categorical or random color scheme.
These are features of equal value but with unique characteristics.

 

The colors you use to visualize your data impact how it is viewed by others.

For this map you are tasked with symbolizing elevation on Mars.

 

 

Working with Color Schemes

 

We have covered how to do this in the past, but this time we will go a little more in depth.

 

1. Click on the color symbol for your layer in the ToC

2. On the right side of your screen the symbology pane will appear.
3. The primary symbology should be set to stretch.

This will give you continuous color schemes.

4. Pick a color scheme you like. I’m going with “Yellow-Orange-Brown”
5. Click on the color scheme tab and select Format Color Scheme

 

 

That’s better but it’s not quite how I want it. You can adjust the little tabs below the color scheme to make changes to it. I removed three of the tabs from the right side of the color scheme. To remove them, just click and press delete. To change the colors of them, just select one, and change the color of the box from the color selector below them.

 

Another way to change the appearance of your color scheme is to use the Appearance tab in the ribbon up top. Go ahead and play around with all of the options

Remember:

DEMs should be displayed over the hillshade. Transparency or the blend modes can be used.

Blend modes can work to preserve the colors in the color ramp a bit more. Transparency can wash out the colors.

Multiply blend mode shown above, 50% transparency shown below:

 

Submission details:

Create a professional figure displaying the Herschel Crater:

• Display the DTM over hillshade (with your custom color ramp)
• You must include the hillshade. Make the DTM transparent or blended so the hillshade shows.
• Simple scale bar - professionally edited
• Round values
• Reduce the number of mid-scale labels.
• Data Credits
• Some kind of explanatory title or figure caption.

 

Example Maps:

     

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