Map Analysis book, Topic 5 Exercises

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Map Analysis: Understanding Spatial Patterns and Relationships

Topic 5. Calculating Visual Exposure — Visual analysis is an extension of effective distance measurement that considers line-of-sight connectivity among map locations. This section discusses the procedures, considerations and applications of derived viewshed and visual exposure maps.

18 Line-of-Sight Buffers Add Intelligence to Maps — describes procedures for creating line-of-sight buffers that track relative visual exposure and noise levels.

19 Identify and Use Visual Exposure to Create Viewshed Maps — discusses basic considerations and procedures for establishing viewsheds and visual exposure from point, line and polygonal features.

20 Visual Exposure Is in the Eye of the Beholder —investigates procedures for assessing simple and weighted visual exposure.

21 Use Exposure Maps and Fat Buttons to Assess Visual Impact — describes procedures for creating a simple model that determines the relative visual impact of alternative power line routes on local residences.

Hands-on Experience

Exercise 5.1 Calculating Viewsheds — in this exercise you will first create a map of all the water locations (viewer map) in the Tutor25 database and then generate a simple viewshed map that indicates the visual connectivity to water— all locations are identified as either 0= not seen or 1= seen from at least one water location.

Exercise 5.2 Calculating Visual Exposure — this exercise demonstrates generating a visual exposure map to water indicating the number water locations visually connected to each grid location in a project area— 0= not seen with increasing values indicating higher visual exposure to water.

Exercise 5.3 Accounting for Screens — this exercise extends the previous exercise to create another visual exposure map to water that accounts for a screening forest canopy of 75 feet and then compares the result to the “non-screened” solution to determine the differences in the two approaches.

Exercise 5.4 Calculating Weighted Visual Exposure — this exercise first calibrates Roads in terms of traffic flow and then creates a weighted visual exposure map accounting for the relative amount of traffic on different road types— 0= not seen from any road location with increasing values indicating higher weighted visual exposure to traffic flows.

Exercise 5.5 Modeling Visual Exposure Impacts — this exercise creates and classifies visual exposure maps for relative connectivity to roads and houses (Low, Medium, High) and then combines the two classified maps into a single map that characterizes the joint visual exposure for each map location using a 2-digit code— a location with a value of 11 indicates 1= Low housing exposure and 1= Low roads exposure; a value of 12= Low/Medium, … to a value of 33= High/High.

Exercise 5.6 Extending Visual Analysis to Other Areas — this exercise creates a visual exposure map to roads and graphically overlays it on the Elevation surface for the Island database.

Cross-Reference for Grid-based Map Analysis Operations used in Topic 5 exercises (a complete cross-reference is on Companion CD)
Class	Description	MapCalc Command	ESRI GRID/SA Command	ERDAS Imagine Command
Reclassify	Assigns new values to the categories on an existing map	RENUMBER	RECLASS	RECODE
Distance	Creates a map indicating areas that are visible from specified locations (visual exposure)	RADIATE (simply)	VISENCODE, VISIBILITY	VIEWSHED ANALYSIS
Distance	Creates a map of the number of visual connections from a set of viewing locations (visual exposure)	RADIATE (completely)	VISENCODE, VISIBILITY (limited visual exposure)	<None, only simple viewshed>
Distance	Creates a map of the viewshed or visual exposure considering screening features (i.e., tree height) on top of the elevation surface	RADIATE (thru)	VISENCODE, VISIBILITY (limited screening capability)	<None, only simple viewshed>
Distance	Creates a map of the sum of the weights of the visual connections from a set of viewing locations (weighted visual exposure)	RADIATE (weighted)	<None, only non-weighted visual exposure>	<None, only simple viewshed>
Overlay	Creates a map as the mathematical or statistical function of two or more maps	CALCULATE (plus)	GRIDMATH	ADD

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Topic 5 Exercises

Access MapCalc using the Tutor25.rgs by selecting Startà Programsà MapCalc Learnerà MapCalc Learnerà Open existing map setà MapCalc Dataà Tutor25.rgs. The following set of exercises utilizes this database.

5.1 Calculating Viewsheds

Use the View button (binocular icon) to select and display the Water map.

Note that there are eight different types of water depending on its flow.

Press the Map Analysis button and choose Reclassifyà Renumber to access the dialog box for reclassifying map values. Complete the input specifications as shown below to derive a binary map of Flowing Water

Select the Water map from the drop-down list, then specify 1 as the NewValue, 1 as the OldValue and 8 as the OldUpperValue. Press the Add button to submit the reassignment phrase. Specify Flowing_water as the new map name and press OK to create a binary map where 1= any water type and 0 = not water.

RENUMBER Water ASSIGNING 1 TO 1 THRU 8 FOR Flowing_water

Press the Use Cells button to set the display type to Grid.

Press the Shading Manager button then enter “Flowing water” as the category description. Double-click on the red Color and choose blue from the color pallet. Click OK to submit the display changes.

Press the Map Analysis button and choose Distanceà Radiate to access the dialog box for visual analysis. Complete the input specifications as described below to derive a binary map of Water_viewshed.

Press the Help button to get a description of the Radiate command’s function and input fields. Specify…

Flowing_water as the viewersMap

Elevation as the surfaceMap

100 as the # of grid spaces away

1 as the viewer_heightValue

Simply as the calculation mode (binary viewshed)

Water_viewshed as the newMap

RADIATE Flowing_water OVER Elevation TO 100 AT 1 NULLVALUE 0 Simply FOR Water_viewshed

Double-click on the map legend to pop-up the Shading Manager dialog box. Enter a description for the map Categories as 1= “Visible from flowing water” and 0= “Not visible.” Note that approximately 95% of the project area is visually connected to flowing water and that the few “Not visible” areas are concentrated along the eastern edge.

On your own, follow a similar visual analysis procedure to generate a viewshed map (Roads_viewshed) of any road location (based on the Roads_type map). What percent of the project area is visually connected to roads?

5.2 Calculating Visual Exposure

Press the Map Analysis button and choose Distanceà Radiate to access the dialog box for visual analysis. Complete the input specifications as shown below to derive a visual exposure map showing “how many” water grid cells are visually connected to every location within in the project area.

RADIATE Flowing_water OVER Elevation TO 100 AT 1 NULLVALUE 0 Completely FOR Water_visualExposure

Selecting the “Completely” calculation mode identifies the number of connected viewer cells. Larger values indicate higher visual exposure to water—locations that “seen” from a lot of water locations (and by direct line-of-sight, “see” a lot from water locations).

Press the Layer Mesh button to superimpose the analysis grid. Press the Use Cells button to set the display type to Grid.

Double-click on the legend to pop-up the Shading Manager. Change the Number of ranges to 12. Note that the data intervals are expressed as increasing steps of 10 additional viewer cells (water locations).

Under the Lock column, click Off the automatically assigned yellow inflection point for the range 30 to 40. Click the Color block for range for 50 to 60 and select yellow from the pallet to reset the color inflection point on the color ramp. Switch the colors for the minimum and maximum ranges by clicking on the respective color blocks and choose green for the lowest range and red for the highest range. Press OK to generate the new display.

Use the Shading Manager to create a display that isolates the areas of very high visual exposure (70 or more water locations visible) as red with a background of grey—set grey as the Color from 0-10 through the 60-70 intervals and Lock on red for the 70-80 and 110-121ranges.

On your own, follow a similar visual analysis procedure to generate a visual exposure map (Roads_visualExposure) of any road location (based on the Roads_type map). What is the highest visual exposure to roads in the project area?

5.3 Accounting for Screens

Use the View button (binocular icon) to select and display the Covertype map.

Use the Map Analysis button and choose Reclassifyà Renumber to access its dialog box and complete as shown.

RENUMBER Covertype ASSIGNING 0 TO 1 THRU 2 ASSIGNING 75 TO 3 FOR Tree_height

Do not forget to press the Add button to enter each Renumber phrase…

ASSIGNING 0 TO 1 THRU 2 (Water & Meadow)

ASSIGING 75 TO 3 (Forest)

…to indicate the height of the vegetation canopy (same units as the Elevation map). This assigns 0 feet (NewValue) for Water (Oldvalue= 1) and Meadow (OldValue= 2) locations and 75 feet (NewValue) for Forest locations (OldValue= 3).

RADIATE Flowing_water OVER Elevation TO 100 AT 1 NULLVALUE 0 THRU Tree_height Completely FOR Water_treesVExpose

Double-click on the legend to pop-up the Shading Manager. Change the Number of ranges to 12.

Change the Calculation Mode to User Defined Ranges.

Starting at the bottom of the Min [>=] column enter values increasing by 10 as shown above, then ending with 121 at the top of the Max [<] column. Set the Color settings the same as before— green for the lowest range, red for the highest range and yellow for the 50 to 60 range. Press OK to display the map using the custom legend that is the same as used for displaying the Water_visualExposure map generated in the previous section.

Click on the Restore Down button in the upper-right corner of the display. Use standard Windows “click-and-drag/size” techniques to position the Water_visualExposure and Water_treesVExposure side by side as shown below.

Note the visual differences between the two maps—visual exposure with and without accounting for the height of the tree canopy. The area of high visual exposure (red tones) that accounts for the tree canopy barriers has a similar shape but is smaller than the corresponding area without trees.

Note: Each map display is contained in a separate window. Standard Windows techniques such as cascading, horizontal and vertical “tiling” are available.

5.4 Calculating Weighted Visual Exposure

Use the View button (binocular icon) to display the Roads_type map.

Press the Map Analysis button and choose Reclassifyà Renumber to access its dialog box and complete as shown.

RENUMBER Roads ASSIGNING 1 TO 1 ASSIGNING 3 TO 2 ASSIGNING 5 TO 3 ASSIGNING 15 TO 4 ASSIGNING 3 TO 21 ASSIGNING 5 TO 31 THRU 32 ASSIGNING 15 TO 41 THRU 43 FOR Roads_relativeVis

Do not forget to press the Add button to enter each Renumber phrase…

ASSIGNING 1 TO 1 (one car)

ASSIGING 3 TO 2 (three cars)

ASSIGNING 5 TO 3 (five cars)

ASSIGNING 15 TO 4 (fifteen cars)

…to indicate the relative number of cars in a given period of time.

Press the Map Analysis button and choose Distanceà Radiate to access the dialog box for visual analysis. Complete the input specifications as shown below to derive a weighted visual exposure map showing the relative visual exposure to roads for every location within in the project area.

RADIATE Road_relativeVis OVER Elevation TO 100 AT 1 NULLVALUE 0 Weighted FOR Roads_weightedVExpose

Note that the weighted visual exposure ranges from 0 to 529. The most visually exposed location is in the west border at column 1, row 20 as reported in the lower-left corner of the display window as the mouse is positioned over the location on the map.

5.5 Modeling Visual Exposure Impacts

Double-click on the Roads_weightedVExpose map’s legend to pop-up the Shading Manager. Select the Histogram tab and click Std. Dev. button to get a plot of the data distribution. Note that the average is about 150.

Based on the Roads_weightedVExpose data distribution, create a map that identifies areas of…

Low= 1 (0-100 seen)

Medium= 2 (100-200 seen)

High= 3 (>200 seen)

…visual exposure by choosing Reclassifyà Renumber and completing the dialog box as shown below.

RENUMBER Roads_weightedVExpose ASSIGNING 1 TO 0 THRU 100 ASSIGNING 2 TO 100 THRU 200 ASSIGNING 3 TO 200 THRU 1000 FOR Road_VExpose_classes

Notice that the area of High visual exposure to roads is concentrated in the northwestern portion of the project area while the areas of Low visual exposure are generally in the southeast.

Use the View button (binocular icon) to select and display the Housing map.

The values stored for grid cell indicates how many houses occur at that location (1 hectare grid cell).