How do you determine whether two proposed microwave relay towers have an unobstructed path to each other? Given a set of locations for fire lookout towers, can the entire forest be seen? How frequently can a proposed disposal site be seen from an existing highway? Solutions to these types of problems are solved with visibility analysis. Two types of visibility analysis are line of sight and viewshed. Line of sight tells you whether a given target is visible from a particular point of observation. It answers the question, Can I see that from here? Viewshed analysis identifies the areas on a surface that are visible from one or more observation points. It answers the question, What can I see from these locations? Determining whether two points can see each other is the most basic function of ArcView Spatial Analysts visibility analysis tools. Visibility analysis is performed with the Line of Sight tool availble in the Visibility Tools sample extension. You can perform viewshed analysis with the Calculate Viewshed option on the Surface menu. The Visibility Tools sample extension also allows you to perform viewshed analysis
Surface analysis tools Table of Contents Topic: Visibility analysis Concepts Calculate viewshed Visibility request Visibility Tools sample extension Line of sight Example Analyzing visibility from a freeway Exercises Calculate viewshed Use the Visibility Tools sample extension Topic: Derive data from surfaces Concepts Deriving slope Deriving aspect Computing curvature Exercise Derive slope, aspect, and curvature Lesson summary Lesson self test Goals In this lesson, you will learn how to: • calculate line of sight • calculate viewshed • derive slope from surface data • derive aspect from surface data • measure curvature of a surface Topic 1: Visibility analysis How do you determine whether two proposed microwave relay towers have an unobstructed path to each other? Given a set of locations for fire lookout towers, can the entire forest be seen? How frequently can a proposed disposal site be seen from an existing highway? Solutions to these types of problems are solved with visibility analysis. Two types of visibility analysis are line of sight and viewshed. Line of sight tells you whether a given target is visible from a particular point of observation. It answers the question, "Can I see that from here?" Viewshed analysis identifies the areas on a surface that are visible from one or more observation points. It answers the question, "What can I see from these locations?" Determining whether two points can see each other is the most basic function of ArcView Spatial Analyst's visibility analysis tools. Visibility analysis is performed with the Line of Sight tool availble in the Visibility Tools sample extension. You can perform viewshed analysis with the Calculate Viewshed option on the Surface menu. The Visibility Tools sample extension also allows you to perform viewshed analysis Calculate viewshed Whenever you need to know which locations can be seen from one or more observer locations, you will want to perform viewshed analysis. If, on the other hand, you want to know how visible the points are from locations specified by a point or line theme you'll perform visibility analysis. Suppose you manage a forest that supports multiple uses. You would perform visibility analysis to check whether drivers along a scenic route could see areas targeted for harvesting. The Calculate Viewshed option on the Surface menu becomes enabled when you make a point or line feature theme and a grid theme active. The point or line features represent the observation points. Different visibility conditions, such as height offsets and angle of view, can be set. The result of a viewshed calculation is a temporary integer grid in which each cell is assigned a visibility code. Cells assigned a value of 0 are not visible from any observation point. Visible cells are assigned a value equal to the number of points from which they can be seen. Calculate Viewshed was used to create this grid theme showing the area visible from a Forest Service observation tower located on Keller Peak in the San Bernardino mountains. [Click to enlarge] Like any grid, a visibility grid can be symbolized to emphasize important information. Left: The grid that results from a viewshed calculation. Surface areas that can't be seen by any observer are symbolized in red. Areas that can be seen by at least one observer are symbolized in green. Right: The visibility grid has been reclassified by the number of observers who can see each cell. Red areas can't be seen by any observers. The visible areas of the grid are ramped from light green (one observer) to dark blue (four observers). [Click to enlarge] By default, observation points are positioned one z unit above the surface and can see to the extent of the surface in all directions with an unlimited vertical angle of view. You can modify these default parameters by adding fields to the observation theme's attribute table. If fields with the specific names listed below (not case-sensitive) are added to the observation theme table, their values will be used as described. Each of the fields is optional and can be used by itself or in combination with others. Individual records can have unique values within a field. In other words, different visibility parameters can be assigned to individual features in the observation theme. (If the observation theme is a line theme, all vertices within a particular feature will have the same parameters.) • SPOT: defines the height of the observation point. If a SPOT field is not present, the surface elevation for the observation points is interpolated from the input surface theme. The SPOT field is useful when all observation points are located at absolute heights. Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: A SPOT field has been added to the observation theme and given values of 3000 meters for each point. (The highest elevation in the grid theme is 1,966 meters.) These values might represent visibility from an airplane at each of the five observation points. [Click to enlarge] • OFFSETA: defines an offset that is added to the surface elevation of the observation points. This is useful for observation points that are located on ground-based towers or platforms. Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: The viewshed has been calculated using an OFFSETA field with values of 30 meters for each observation point. [Click to enlarge] • OFFSETB: defines an offset height that is added to the target surface cells. This is useful for evaluating the visibility of objects with known heights. Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: The viewshed has been calculated using an OFFSETB field with values of 50 meters for each record (as if each cell contained a tower 50 meters high). [Click to enlarge] • AZIMUTH1 and AZIMUTH2 set horizontal angle limits to visibility. The scan proceeds in a clockwise direction from AZIMUTH1 to AZIMUTH2. Values are given in degrees from 0 to 360, with 0 oriented to the north. Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: AZIMUTH1 and AZIMUTH2 fields have been added to the observation points theme table. For each point, the AZIMUTH1 values are 90 and the AZIMUTH2 values are 270. Each observer therefore has a field of vision of 180 degrees, looking southward. (The observation points are shown for reference.) [Click to enlarge] • VERT1 and VERT2 set vertical angle limits to visibility. VERT1 sets the upper limit to the scan (maximum 90 degrees) and VERT2 sets the lower limit (maximum -90 degrees). The horizontal plane (0 degrees) is defined by the z value of the observation point plus the value of OFFSETA. Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: This grid has the same OFFSETA and OFFSETB values, but VERT1 and VERT2 fields have also been added. The VERT1 value is 90 degrees (the default) for all points. The VERT2 value, however, is 0, which eliminates all field of view below the level of the horizon. [Click to enlarge] • RADIUS1 and RADIUS2 limit the visible distance from each observation point. Areas outside the RADIUS2 search distance are excluded from the analysis. Areas inside the RADIUS1 search distance can't be seen in the visibility grid, but may still block the visibility of cells between RADIUS1 and RADIUS2. Left: An elevation grid theme and a point theme. Each of the points represents the location of an observer. Right: A visibility grid created with RADIUS1 values of 1,000 meters and RADIUS2 values of 2,000 for each point. [Click to enlarge] • By default, the RADIUS1 and RADIUS2 distances are interpreted as 3D line of sight distances. The values can be processed as 2D planimetric distances by inserting a minus sign in front of the numbers. Determining visibility is a computer-intensive process. The smaller the grid resolution (cell size), the longer the process. During your preliminary examples, you may want to set the Analysis Properties for Cell Size larger. For a final study, you can reset the cell size small enough to meet the study's requirements Visibility request Use the Visibility request to analyze visual exposure and to perform viewshed analysis. Avenue syntax: aGrid.Visibility(anFTab, aPrj, cellObserved) Visibility is calculated for each cell of a grid theme in relation to features found in anFTab (a feature table). If cellObserved is true, the output grid will contain fields in its value attribute table (VAT) for each observer point containing information about whether that cell can be seen by each observer. If cellObserved is false, each cell in the output grid will contain the number of observers that can see that point Visibility Tools sample extension Many sample extensions come with ArcView. These extensions are located in the samples subdirectory of the directory where ArcView is installed. One of these is the Visibility Tools sample extension. After loading the extension, you will notice two new tools on the view GUI: the Line of Sight tool and the Visible Area tool . The Line of Sight tool allows you to drag a line of interest on the view. The result is a line of sight graphic on the view. Sections of the line that are visible from the starting point are marked in green, and sections that are not visible from the start are marked in red. With this tool, you can control target and observer offset. This would be important if you needed to find out which areas are visible from a fire lookout tower that is 15 meters off the ground. The Set Visibility Parameters dialog. [Click to enlarge] The Visible Area tool returns a polygon feature theme of the area that is visible from the starting point of the line. A polygon theme of visible areas from the starting point of the line in the upper portion of the view. [Click to enlarge] You set observer and target offsets, the field of vision, and the near and far distances. If your analysis requires a grid, you can convert the visibility theme to a grid. The Set Visibility Parameters dialog. [Click to enlarge] Sample extensions are included to help you successfully use ArcView. Note, however, that these samples are not supported by ESRI Line of sight Line of sight analysis determines whether a given target is visible from a given observer's point of view. A line of sight is drawn with the Line of Sight tool between an observer and a target (represented by the starting and ending points of the line). A grid theme must be active in a view for the tool to be enabled. A line of sight is a grouped graphic. Portions of the terrain along the line of sight visible to the observer are drawn as green line segments; invisible portions are red line segments. A line of sight was drawn from the lower left to the upper right over the mountain. Green line segments indicate visible terrain along the line graphic. Red line segments indicate terrain that is not visible. [Click to enlarge] The visibility analysis assumes that a straight line is drawn between the observer and the target. It follows that a target may be visible even though the line segment leading to it is red (if, for instance, the target has a sufficient height offset). It also follows that when the view of a target is obstructed, the obstruction point will lie on a visible segment of the line of sight. These points are more easily appreciated from profile graphs. Line of sight visibility profile for the line drawn across Mt. Ranier. You can see the profile of the mountain. Green line segments represent visible terrain. Red line segments represent hidden terrain. [Click to enlarge] Offset heights for both the observer and the target can be set in the Set Visibility Parameters dialog. This dialog opens when the Line of Sight tool is selected. The offsets remain in effect for all lines drawn with the tool until another tool is made active. (Clicking on the tool at any time opens the dialog and allows you to change the offsets.) The Set Visibility Parameters dialog. [Click to enlarge] The offset heights are in the map units of the active surface theme Example Analyzing visibility from a freeway Bert is a GIS analyst for a state transportation department. A freeway beautification project, to consist of a steel and concrete sculpture and a waterfall, has recently been approved. It doesn't sound that beautiful to Bert, but his job is simply to evaluate the suitability of four potential sites along a particular stretch of freeway. The themes drawn in the view are an elevation grid theme (Terrain), a streets theme, a freeway theme, and a point theme of potential sites for the beautification project. The sites rank equally well in all respects except visibility, the last important factor to be analyzed. The site that will be selected is the one that can be seen from the most number of locations on a seven-mile section of the freeway. To solve the problem, Bert does a viewshed calculation. As his surface theme, Bert uses a grid theme of the local terrain. The Freeway line theme could be used as his observation theme, but the line has so many vertices that the analysis would be extremely slow. Instead, Bert creates a new theme of observation points spaced along the freeway at quarter-mile intervals. By default, ArcView Spatial Analyst assumes that observers are located one z unit above ground level and that targets are located exactly at ground level. (The z units for this data are feet.) Bert modifies the defaults by adding fields to the observation theme table. He adds an OFFSETA field and assigns a value of 3 to each observation point. This will locate the observers three feet above ground level, about the height of a driver. He adds an OFFSETB field and gives a value of 50 to each observation point. This means that each observer is assumed to be looking at an object 50 feet high (the height of the proposed sculpture). Other parameters, such as the observers' horizontal and vertical angles of view, are left to their default unrestricted values. Top: A zoomed-in view of a section of freeway. Each red dot is an observation point. The two blue squares are proposed sites. Bottom: The observation theme table. The OFFSETA field sets a height above ground level for each observer. The OFFSETB field sets a height for the target. Bert makes the Observation points and Terrain themes active, then chooses Calculate Viewshed from the Surface menu. A temporary integer grid is created and added to the view. The grid displays visibility for the entire surface from the freeway. Green represents visibility and red non-visibility. In this analysis, if a cell is green, it means that a 50-foot high object located in that cell can be seen from at least one of the 29 observation points along the freeway. If a cell is red, a 50-foot high object located there cannot be seen from any of the observation points. The grid theme table correlates the number of cells (Count) to the number of observers (Value) who can see them. Left: The visibility grid output from the viewshed calculation. A 50- foot tall object located in any green cell can be seen from at least one observation point. In red areas, the object would not be visible from any observation point. Right: The visibility grid theme table. 2100 cells are seen by zero observers, 352 cells are seen by one observer, 405 cells by two observers, and so on. In this particular analysis, the maximum value is 26. There are 11 grid cells (out of a total of 16,124) that can be seen by 26 of the 29 observers. All four potential sites are located in green areas, which means they are visible from at least one point on the freeway. But what Bert wants to know is which one can be seen from the most number of points. To find out, he makes the visibility grid theme active and clicks the Identify tool. Then he zooms in on the proposed sites and clicks on them. The Identify Results windows for each of the four potential sites. The Value field tells you how many observers can see the identified location. The first site can be seen from 15 of the 29 freeway observation points, the second (moving from west to east) from 13, the third from 21, and the fourth from 9. [Click to enlarge] The best of the potential sites can be seen from 21 of the 29 observation points. Of the four potential sites, the highlighted one can be seen from the most freeway locations Exercise Calculate viewshed In this exercise, you will use ArcView Spatial Analyst's Calculate Viewshed function to choose between five alternative microwave relay station sites. The best site will be that which has line of sight visibility to the greatest portion of the study area. The five candidate sites are all located on peaks in the mountains to the north. If you have not downloaded the exercise data for this module, you should download the data now. [...]... your surface- based criteria Again, the criteria were made up just for this exercise Step 12 Close the project Close the project without saving any changes You have completed this exercise Treat yourself to an orange Summary In this lesson, you learned about some of ArcView Spatial Analyst's surface analysis tools Visibility analysis includes line of sight analysis and viewshed analysis Line of sight analysis. .. Visibility Analysis view containing an Elevation grid of Mt St Helens Step 3 Load the Visibility Tools sample extension This exercise requires the Visibility Tools sample extension You'll load it now To load the Visibility Tools sample extension in ArcView, make the Project window active, then choose Extensions from the File menu Check the box next to Visibility Tools (sample) and click OK If Visibility Tools. .. green areas are depositional surfaces and the red are erosional The white area near the center is the sand and gravel quarry in the Santa Ana River wash The bottom of the quarry is flat, so there is no change in slope Step 10 Create a grid of suitable depositional surfaces Now you'll use Map Query to create a grid of depositional surfaces for the orange tree model From the Analysis menu, choose Map Query... are flat, and thus have no aspect, are assigned a value of -1 Curvature is a measure of a shape for surface A positive curvature indicates that the surface is upwardly convex at that cell A negative curvature indicates that the surface is upwardly concave at that cell A value of zero indicates that the surface is flat Deriving slope Slope is calculated as the maximum rate of change between a cell and... curvature of a surface at each cell center, and optionally, the slope and aspect This is the Introduction to Surface Analysis with ArcView Spatial Analyst - Lesson 3 Self test Please watch your time—you have 2 hours to complete this test Use the knowledge you have gained in Introduction to Surface Analysis with ArcView Spatial Analyst to answer the following questions You will need to correctly answer... Curvature request returns up to five grids that are measures of the behavior of the surface The output curvature grid is the rate of change of the slope (slope is the first derivative of the surface, curvature is the second) The profile curvature grid is a measure of the acceleration or deceleration of flow over the surface and may be used as a measure of erosion and deposition The planform curvature... Derive data from surfaces How do you determine the location of steep slopes to help predict avalanche risk? How do you determine north and south facing mountain slopes to help predict seasonal snow melt? Where are areas on a surface likely to hold water in the event of a major flood? The answers to these questions can be found by deriving grids of aspect, slope, and curvature from surface data Slope... the Select Record tool Towercov.shp theme table and select the first record in the Attributes of Make the Microwave Relay Visibility Analysis view active Make both the Towercov.shp and Elevation themes active From the Surface menu, choose Calculate Viewshed Visibility analysis is often very time-consuming, so be patient while the new grid is being calculated The result is a new theme called Visibility... Calculate Slope option on the Surface menu or by using the Slope request Aspect identifies the downslope direction of a cell and is measured in compass degrees ranging from 0 to 360 Aspect is calculated in ArcView Spatial Analyst by using either the Derive Aspect option on the Surface menu or by using the Aspect request The Curvature request calculates the curvature of a surface at each cell center,... (oranges like lots of sunlight) Depositional soils (depositional areas are exposed to surface water longer and generally consist of welldrained sandy soils) You will use ArcView Spatial Analyst's surface analysis tools to create grids for each of these criteria and use them in a model at the end of the exercise If you have not downloaded the exercise data for this module, you should download the data . Surface analysis tools Table of Contents Topic: Visibility analysis Concepts Calculate viewshed Visibility request Visibility Tools sample extension Line of sight Example. Analyst's visibility analysis tools. Visibility analysis is performed with the Line of Sight tool availble in the Visibility Tools sample extension. You can perform viewshed analysis with the. sight • calculate viewshed • derive slope from surface data • derive aspect from surface data • measure curvature of a surface Topic 1: Visibility analysis How do you determine whether two proposed