GIS

Geospatial Analysis with AutoCAD Map 3D

19 Aug, 2009 By: Michael Schlosser

Engineering software offers GIS querying and analysis capabilities that take you beyond data management


Autodesk's AutoCAD Map 3D is widely known as an engineering program for creating and managing CAD and spatial data. However, did you know that AutoCAD Map 3D also has querying and geospatial analysis capabilities?

Text Box: Uses of geospatial analysis include:  •	Bills of materials  •	Scheduling  •	Notification requests  •	Budgeting  •	Preplanning  •	Feasibility studies  •	Utility design  •	Site selection  •	Shadow studies  •	Surface studies  •	…

That’s right! AutoCAD Map 3D is a true engineering GIS software program: a program that embraces CAD and geospatial data; a program that integrates this data in a manner that respects both engineering design and GIS requirements; a program that not only helps you create and manage both types of data, but also helps you analyze this data.

Whether your focus is CAD or GIS, geospatial analysis can help you derive more value from your data and aid in better decision making. Whether you are creating a bill of materials, designing a new utility network, or conducting a feasibility study, geospatial analysis is an important addition to your CAD and GIS skill set.

This article introduces some of the geospatial analysis capabilities of AutoCAD Map 3D by addressing the role of queries, thematic mapping, buffer analysis, topology analysis, and 3D surfaces in analyzing your CAD and GIS data. In future articles in this series, I'll offer more detailed guidance to help you make the most of these tools.

In this example, a query is used to identify all cast iron water mains.

Queries
The geospatial query is a simple, yet powerful geospatial analysis tool. Queries are used to retrieve a subset of objects or features by attribute and/or location. They help you to limit the data that you’re working with, isolate specific objects and features, and otherwise discover patterns in your data. Queries help you to answer questions such as:

•   Where are all the cast iron water mains greater than 50 years old?
•   What is the total length of overhead conductor in a subdivision?
•   How many blocks named METER are in my drawing?

Whether you are working with CAD data (such as DWG files) or feature data (including GIS data sources), AutoCAD Map 3D supports the ability to query your data by specific attribute information (e.g., material type = "cast iron") and/or geographic location (e.g., number of meters within a specific area).

Thematic Mapping
Thematic mapping uses display properties such as color, line type, and symbology to visualize attribute data corresponding to specific map features and to help identify and compare spatial patterns. For example, the common road map that you can purchase from your local gas station is a thematic map that uses different line styles to help distinguish the various road types such as highways, collector roads, and local streets. Another common example is the municipal zoning map in which areas are color-coded according to their zoning designation, such as residential, commercial, and industrial.

Attempting to create a thematic map in CAD requires separating your CAD entities into different layers — a lengthy and error-prone process. However, thematic mapping is faster and easier when you use the built-in thematic mapping functionality in AutoCAD Map 3D.

Use Thematic mapping to quickly call out various types of CAD entities in a map.

Theme your point, line, and polygon features and control color, line style, symbology, and polygon fill styles to create thematic maps even if your data resides in external data resources such as ESRI SHP files or Oracle databases. You can also generate a corresponding legend with the built-in legend creation tool.

 

Buffer Analysis
A buffer is a polygon that is created at a specified distance from a feature or set of features. For example, a 30-foot buffer around a point is a circle with a 30-foot radius. Buffers are used to determine proximity and to select other features for reporting or additional analysis. For example, municipal rezoning applications typically require that all affected property owners be notified of the proposed change in zoning. Affected properties are identified by determining which parcels lie within a stipulated distance (the buffer distance) of the parcel being considered for rezoning. These parcels and their corresponding address information are then used to generate the rezoning notification.

 

Here, a buffer identifies parcels adjacent to proposed road construction.

To create a buffer around a set of lines or polygons in CAD, you could offset the required linework and then use the resulting lines to manually piece together the polygon representing the buffer — a tedious and time-consuming process. AutoCAD Map 3D helps simplify the creation of a buffer with the assistance of a dialog box that allows you to select the entities to be buffered, specify the buffer distance, and choose how the resulting buffers are merged. The resulting buffer can then be used in a geospatial query or other types of geospatial analysis.

 

Topology Analysis
Topology records the geospatial relationship between points, lines, and polygons. With topology you can find the answers to questions such as, What is the shortest travel distance from point A to point B? and How many hydrants are within a specific pressure zone?

An overlay identifies parcels based on proximity to waterways and majopr transportation corridors.

Topology analysis requires clean data. For example, polygons must be closed and consist of lines that do not overlap or intersect. AutoCAD Map 3D includes drawing cleanup tools that help you find and correct potential topology errors such as duplicate objects, gaps in your linework, and intersecting lines. Once your data has been cleaned, you can create node (i.e., point), network (i.e., line), and polygon topology from your CAD data — or simply use existing GIS data sources.

 

AutoCAD Map 3D supports a variety of methods for analyzing topology, including network and overlay analysis. Network analysis is used to perform flood-tracing operations that help solve line connectivity questions and uses resistance values, such as travel distance, to compute shortest and best paths. Overlay analysis is used to determine the spatial relationship between different topologies. The method accepts two input topologies to create a third based on a stipulated overlay operation such as intersection, union, and clip. For example, an intersect overlay operation can be used to find all parcels that fall within 100-year flood zones. The resulting topology includes features from the areas common to both source and overlay inputs and contains attributes from both.

3D Surfaces
AutoCAD Map 3D supports the analysis of 3D surfaces such as DEMs (digital elevation models), DTMs (digital terrain models), and ESRI Grid files. With the Surface Creation Extension for AutoCAD Map 3D software, you can also create surfaces from point data collected from surveying and GPS instruments, AutoCAD points and blocks in drawings, and point and contour data residing in external data sources such as ESRI SHP files.

Parcels themed by elevation shown draped over a 3D surface.

You can use AutoCAD Map 3D to create contour maps, as well as to stylize, theme, and analyze surfaces by elevation, slope, and aspect. Automatically drape (or superimpose) your two-dimensional CAD, GIS, and aerial imagery onto your surfaces, and visualize the combination in 3D. You can use hill shading to enhance the visualization of 3D surfaces in plan view, as well as perform 3D shadow studies based on various sun properties, geographic locations, and times of day.

The ability to analyze 3D surfaces is useful for performing preliminary site suitability studies. For example, with AutoCAD Map 3D you can theme your 3D surface by aspect and then drape your 2D GIS-based parcel data over the resulting surface to help you visually determine which parcels are predominantly south-facing. 

In-Depth Analysis
This article has explored some of the geospatial analysis capabilities of AutoCAD Map 3D. Engineering, CAD and GIS professionals, and anyone interested in bridging the CAD/GIS gap can benefit from learning how to analyze their CAD drawings and geospatial data using the analysis techniques discussed. Watch for future articles where I will expand on each of these capabilities.


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