Building a Site Drawing with GIS (From the Trenches AutoCAD Tutorial)1 Jan, 2008 By: Salvatore Napolitano
Use AutoCAD Map 3D and GIS data to create accurate site plans in a short time.
Building a preliminary site drawing from GIS will expedite your work because you can develop a conceptual site plan while waiting for the surveyor, wetlands scientist, and geologist. In addition, the conceptual drawing can provide an early warning of what may be found when the site is investigated. A GIS plan will disclose areas of concern within the site.
You can look at a blank page or you can go the Web and build on an existing site plan and draft in your proposed tanks, buildings, and pipelines. But without GIS you will not know whether to expect groundwater at four feet or at 400 feet! That may put you at a disadvantage. Where are the wetlands? Do you expect to find rock, and how deep is it to bedrock? These are just some of the data you can add to your drawing while sitting at your keyboard.
How Other Firms Are Using GIS
In "GIS for Site Planning" (CENews.com, July 1, 2006), James J. DeVellis illustrates a variety of ways in which GIS is helping speed site analysis and design. Here are three examples:
- By using GIS to create a plan for Fitton Field in Worcester, Massachusetts, site planners shaved three months from the project schedule. A subsequent field survey showed that the GIS data was highly accurate.
- When GIS was used in planning a 150-acre senior campus community in Dedham, Massachusetts, it uncovered rare vegetation in a remote area of the site that may not have been discovered during the site walk. This knowledge allowed planners to design around and preserve the vegetation.
- GIS also allows people who are unfamiliar with reading traditional site plans to better understand the project through clear graphic representations of a site. The GIS maps are useful for presenting a project to town officials and abutters who, in many cases, can be a deciding factor in the fate of the project.
GIS is approximate and most useful for large sites. As a conceptual drawing for a large site, it is the way to go.
Where To Find All this Data
You can search Google to find state and federal agency GIS Web sites; you can also go to a Web site that has GIS links sorted by national and state organizations and in alphabetical order. One such site is the University of Arkansas library's "Guide to Mostly On-Line and Mostly Free U.S. Geospatial and Attribute Data."
Many types of data are acquired using GIS technology. Below is a list of some available types that engineers and site planners will find useful:
- aerial photographs
- U.S. Geological Survey (USGS) maps
- water table
- endangered species
- ground elevation
- ground cover
- existing pipelines and underground tanks
- American Association of State Highway and Transportation Officials (AASHTO) soil classification
Getting GIS Data into Your CAD Drawing
With Autodesk Map or Map 3D, set your drawing zone to the coordinate system you want to be in. I usually use State Plane NAD83 Feet for my projects. The aerial photos are usually in TIFF or MrSID format and can be downloaded with an associated file that will insert the image at your site's coordinate system. Physical data -- such as town boundaries, wetland, or soil delineations -- are shape files. Table data is usually Microsoft Access or XBASE (.DBF). In this example I am using the site for a wastewater treatment plant and piping that I worked on in Provincetown, Massachusetts.
If you are not familiar with Autodesk Map or Map 3D, you can start with the tutorials that come with the program. The Autodesk Web site has the tutorials in a free 140-page PDF.
You have access to about 57,000 USGS 7.5 minute quadrangle maps. I recommend downloading an index for the state and then querying the sheet ID as text (shown in red in the figure below). I frequently use these AutoCAD drawing indexes as overlays to determine which quadrangles I need. Usually you will be linked to an FTP site with hundreds of numbers. With this CAD overlay drawing you will know which orthoquad numbers to download for your site. As you can see in the first figure, these are not USGS sheet sizes, but smaller areas.
A sample USGS orthoquad index of Provincetown, Massachusetts, with the USGS sheet name in black and the orthoquad sheet number in red.
The USGS map shown above was in Massachusetts State Plane NAD83 Meters before Autodesk Map inserted it into my coordinate system. The maps may be old, but I still find them useful for orienting myself to the site and its surroundings.
You can download an aerial photo from a state GIS site using the same sheet IDs that you used with the USGS orthoquads. On the Massachusetts Geographic Information System (MassGIS) Web site you can select aerial photos from different times. You can see that it is updated much more frequently than are the USGS maps. The MassGIS Web site offers aerial photos from several different years. Below are examples from before and after the construction of the example site in Provincetown.
Wetlands delineations are downloaded from the MassGIS Web site and imported into Autodesk Map with a coordinate transformation from meters to feet NAD83 State Plane.
Wetlands comparison of GIS data and wetland flags with the GIS wetlands shown in blue hatch. (Click image for larger version)
As the above figure shows, GIS indentified the wetlands located on either side of the treatment plant. After our wetland scientist flagged the wetlands, our site plan was still beyond the wetlands and the 100-foot buffer (not shown), clearly illustrating the value of GIS wetlands data for conceptual planning.
Ground Elevation (Topo)
Existing grade contours show elevation in meters at an accuracy of +/-1.5 meters from the MassGIS site. Again you must convert to State Plane NAD83 Feet.
Once you have created your surface with GIS contours, you can drape the aerial photo onto the surface with Civil 3D 2008 as shown in the image below. That is an impressive enhancement to Civil 3D!
With Civil 3D, you can also use Google Earth to acquire terrain images. See "Harness Google Earth Tools in Civil 3D 2008, Part 1," by Mike Choquette, Cadalyst.com, July 1, 2007.
Soils and Water Table
Soils data can be downloaded free from the Natural Resources Conservation Service (NRCS) Web site. The data is in State Plane NAD83 meters.
Below is a list of some NRCS soils data that is important to engineers:
- soils highly corrosive to concrete or steel
- AASHTO group number
- areas prone to high shrinking and swelling
- areas prone to high erosion
- water table depths
After you have your shape files and data tables, you can perform queries to create hatch patterns, etc., to make a thematic map. (With Map 3D you can create themes in the Toolspace.) As an example, I queried soils with high and low corrosion to concrete from the component table. I had to link this table to a map unit attribute in my soil polygons to perform this query. Don't worry if this sounds confusing, I did not have a clue on how to do this before reading tutorial books and downloading tutorials from Autodesk. Persistence will get you there and it is definitely worth the effort. The GIS soils data from NRCS can be very complicated because of components and layers within the group.
The Internet is a great source of free GIS data. As a designer, I use GIS to "see" a potential site and its properties. Operating without it is like operating with my eyes closed. You will find, as I did, that it is not only useful, but also enjoyable and gratifying to create colorful and data-filled drawings after starting from a blank page.
About the Author: Salvatore Napolitano
In her easy-to-follow, friendly style, long-time Cadalyst contributing editor Lynn Allen guides you through a new feature or time-saving trick in every episode of her popular AutoCAD Video Tips. Subscribe to the free Cadalyst Video Picks newsletter, and we'll notify you every time a new video tip is published. All exclusively from Cadalyst!