Unit 6 – Lesson 5: Spatial Analysis Civil 3D 2010 Student Workbook ▪ 1 Spatial Analysis Overview Spatial analysis is the process of working with geospatial data layers to create new data layers in order to answer questions and make decisions during the engineering planning project phase. One of the most common operations is to create a buffer around a feature and then identify features on other layers based on their proximity to the buffer. Another useful method of spatial analysis is called an overlay. Overlays combine two geospatial layers to create a new layer. In the following illustration, the blue buffer defines an area within 100 feet of the river. This buffer could be used to identify the land parcels that lie within the flood zone. Lesson 5 AutoCAD Civil 3D 2010 Education Curriculum Student Workbook Unit 6: Geospatial DataUnit 6 – Lesson 5: Spatial Analysis Civil 3D 2010 Student Workbook ▪ 2 The following illustration overlays two layers, a flood zone and a parcel layer, in order to determine where the two intersect. The intersected parcels layer becomes a new layer, which can be styled and used for additional analysis. Objectives After completing this lesson, you will be able to: - Describe buffer and overlay analysis methods. - Analyze data using a feature buffer. - Analyze data by creating a feature overlay. Exercises The following exercises are provided in a step-by-step format in this lesson: 1. Create a Buffer Analysis 2. Create an Intersection Overlay Analysis About Buffers A buffer is a polygon that is drawn at a specific distance around a selected feature. It is typically used as a way to select features on the map for further analysis. Buffers can be created around points, lines, or polygons. In the Create Buffer dialog box, you specify a distance value for the buffer. If the selected feature is a point, then the distance is the radius of a circle around the point. Spatial analysis on a point buffer feature could be used to identify the number and location of wells within a specific distance from a chemical spill. If the selected feature is a line or polyline, then the buffer value represents an offset distance for the line or polyline. Spatial analysis on a line, polyline, or polygon buffer could be used to identify residential properties affected by specific noise decibel levels adjacent to a proposed runway expansion at an airport. Buffers for multiple features can overlap one another. The following illustrations show a polygon buffer on the left, and a polyline buffer on the right.Unit 6 – Lesson 5: Spatial Analysis Civil 3D 2010 Student Workbook ▪ 3 The buffer is created as a polygon on its own layer. By default, it is styled to be semitransparent and is saved externally as an SDF file. The following illustration displays the result of a buffer operation around a city point, using a distance of 500 miles.Unit 6 – Lesson 5: Spatial Analysis Civil 3D 2010 Student Workbook ▪ 4 Overlapping Buffers A more complex analysis involves using multiple buffers for multiple sources of data such as points and lines. Suppose you are researching seismic activity in the western U.S. You have already created a map (shown in the next illustration) that shows all the earthquakes of a magnitude 6 or more (the red circles), the municipalities with a population of 100,000 or more (the yellow circles), and the major fault lines. To produce this map, you combined property and location queries and display the data from various sources. You are required to identify and display the municipalities in the Los Angeles area that are at the greatest risk, based on past earthquake activity and the proximity of the municipalities to fault lines. The first analysis step is to create a point buffer layer to show the areas within a specific distance of a past earthquake, say 10 miles. The buffers around the individual earthquake points can overlap to form continuous zones, as shown in the next image on the left. The same procedure performed with the fault lines creates a polyline buffer layer (shown on the right).Unit 6 – Lesson 5: Spatial Analysis Civil 3D 2010 Student Workbook ▪ 5 Once the buffer zones are created, a multiple-location query can be used on the municipalities layer to isolate those municipalities that are within one or more of the buffer zones. These municipalities are located within 10 miles of a past epicenter or within 10 miles of a major fault line. Overlay Analysis An overlay analysis is very useful to create new feature layers from existing feature layers. Overlay analysis compares two layers that are spatially related. One layer is designated the source layer, and one is designated the overlay layer. The overlay operation produces an output layer that is saved as an external SDF file. The features and attributes of the new layer vary, depending on the type of overlay operation you perform. A typical application of a type of overlay analysis method is shown in the following illustration. The top image displays a city map with contours. To create a new layer of contour polylines that are located inside the parks and areas of open space in the city, an overlay analysis is used. The source layer is the contour data, and the overlay layer is the parks data. This overlay analysis type is called an intersect. The result is a new layer (an external SDF file), containing only the contours within the boundaries of the parks, as shown in the second image.Unit 6 – Lesson 5: Spatial Analysis Civil 3D 2010 Student Workbook ▪ 6 The ability to restrict the display of features is very useful. However, the main advantage of using the overlay method is to create a new layer based on the analysis. For example, in the following illustration, the transparent blue polygon represents a flood plain. With an overlay analysis, you can easily isolate the features. In this case, the isolated features are the parcels that are inside, or touching any part of, the flood plain.Unit 6 – Lesson 5: Spatial Analysis Civil 3D 2010 Student Workbook ▪ 7 Source and Overlay Layers The geometry in the layers that are selected as the source and overlay layers determines the result
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