| IMPORT VECTOR-BASED CAD SURVEY FILES TO ArcGIS | |
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Joshua Raub American River College, Geography 350: Data Acquisition in GIS; Fall 2005 Contact Information: Golder Associates 1009 Enterprise Way, Suite 350 Roseville, CA 95678 916-786-2424 jraub@golder.com |
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ABSTRACT Survey or topographic information based on vector-based files such as AutoCAD can be imported into ArcGIS without the need of third-party software. This report demonstrates how to import an AutoCAD layer to an ArcGIS shapefile for future GIS use. The importation thru ArcGIS takes time and understanding of the layers along with certain difficulties. |
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INTRODUCTION Engineering firms have used surveyors for providing topographic information to their design. The survey topographic information can come in variety of forms. Several forms of survey results are GPS survey and photogrammetric method. To make the survey information accessible to others, it must be converted to a format. One of several formats used is using computer-assisted drafting (CAD) program such as AutoCAD which is dwg format. The topographic information is mainly in vector-based (lines) instead of raster-based (images). With a growing number of engineering firms using geographic information system (GIS) such as ArcGIS, there will be times where one have to import a CAD survey file into the GIS. While there is third-party software that makes the importation with ease, this paper will demonstrate the basic importation of CAD using ArcGIS without any extra cost beside the time used by the operator of the programs. |
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BACKGROUND There is several third-party software out on the Internet that can do CAD-to-GIS conversion simpler than using straight forward from ArcGIS. One of these companies, guthrie CAD::GIS, offer a program, CAD2Shape (Version 2.0 as of this writing), to convert AutoCAD into ArcGIS shapefile format. While these programs can offer ease of conversions, they can be pricey. The cost for CAD2Shape is $144.50. There are also companies out there can do conversion works for a business so a business can be free to focus on more immediate work. One example of such a business is GeoVect. As again, this kind of service can be costly, depending on how much conversion work needed. ArcGIS do have a basic importing capability for AutoCAD dwg files. There is no additional cost within this conversion beside the time spent by an operator. For this report, the author convert two dwg files: the regional topography of Owen Lake in southern California and a smaller localized topography within the Altamont Pass area near Tracy. |
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METHODS To import a vector-based CAD file into ArcGIS, the author has to edit the layers within the CAD files thru AutoCAD such as checking the lines for proper line types and elevation along with appropriate layer names. Another factor during this portion is to verify the coordinate information. The Altamont Pass information is based on the State Plane System. Since the Altamont Pass topography is obtain thru a private survey company, Walker & Associates, the layers do not conform any standards between companies. Therefore, a company receiving such a survey file may have to "standardize" the file to their own standards before further use. Here is a screenshot of the Altamont Pass topography file within AutoCAD after the author standardized it. The layers within the Altamont Pass were set up in groups that can use for importing and turning on/off. The Owen Lake topography file obtained from the Great Basin Air Pollution Control District and it cover a larger survey area, which results in a larger file size. Its coordinate system is the UTM Zone 11 Feet. One plus of this public domain file is there a file that explains what the layers within the dwg file are. The AutoCAD screen shot of the Owen Lake is shown here. Opening ArcMap, the author creates a new map file (mxd) and open to the folder containing the dwg files. Two different icons show up to open the file with the same name, one contain just the drawing as a CAD drawing type and the other contains point, line, polygon, multipatch, and annotation (text) layers (CAD Feature Dataset) as shown here (the CAD drawing being highlighted). Opening the dwg file (or the CAD drawing file type as noted in picture above) that does not contain the layers would result in showing the drawing as it appears in AutoCAD but no additional layers (screenshot). This is not what the author seeking to import since there are no layers to chose and edit. Rather than using the CAD dwg file, the author use the same drawing name but the CAD feature dataset type, which reveals the point, line, polygon, and annotation layers, seen here. When importing the layers, a warning dialogue would show up that the layers are missing spatial reference information. This will be explained later on. After importing the layers into an ArcGIS mxd file as layers, the screenshot look like this. The author notices that some of the separate layers found in AutoCAD were bundled together. This is when the author has to extract the features from the layer (for example, the polyline layer). Using the Altamont Pass topography, the author shall demonstrate extracting the layer to its own layer. The Altamont Pass topography has numerous attributes under the polyline layer, which is found by right-click on the layer. A screenshot of the attribute table under the layer show the attribute information for each feature. These attribute are based on what information provided by AutoCAD. Information contains layer name, color type, line type and elevation. Using the information based on the AutoCAD layer, a SQL query search under the option pull-down menu can be conducted on which layer to export. For this example, the author search for any layer name matches "TOPO-INT" which stand for intermediate topographic contours. The results then are highlighted, which 17,839 out of 43,101 records matching "TOPO-INT", and ready for extraction from the polyline layer. The author turn off other layers, those are not part of the extraction. Right-click on the polyline layer and select "Export Data" under the Data menu. The author made the shapefile name to "2004_Topo_Int" since it is possible later on to compare different topographic information. After exporting the layer, the author adds the new layer to the mxd file. After importing each layer, the layer can be customized like colors and symbols to the preferences set forth by either the operator of the program or the company. The results of Altamont Pass topography after importing layers is shown here. With these steps, the author repeats again for each layer to export as a new shapefile layer. This could take from a few minutes to several hours, depends on which layers to import and the number of records to chose from (the more records to deal with, the more processing power needed to extract). Owen Lakes was similar to the Altamont Pass topography except that it took longer to export the CAD layers since there were more records to deal with and more complicated layer arrangement. The layer arrangement was a bit easier than it looks due to a Word document describing the AutoCAD layers as well as other important information such as coordinate system and date of topography. Despite of the Word document, it took the author about five to six hours to import the Owen Lakes topography versus two to three hours for Altamont Pass topography. |
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RESULT After several hours on both files, the author created a simple topographic map for each of the AutoCAD files. The Altamont Pass took about three hours from start of importing to finish of layers and another hour or so to set up the map layout. The Owen Lake topography map took longer with about five to six hours on importing and about an hour for the map layout. The PDF version of the finish maps can be found in the Appendices section of this report. |
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CONCLUSION The AutoCAD importation thru ArcGIS is very useful for basic needs without having to use either third-party softwares or an outside company to convert the data. The importation of points and polylines are simple but the author express difficulty importing the text information such as control point information and index contour elevation. These are found under the annotation layer within the CAD Feature Datasheet. Due to given time available, the author could not export the annotation data properly to be used as a separate shapefile or table. This could be a difficult process that could be solved using a third party software or more experience on the importation process. Despite of the difficulty of the annotation importation, the annotation layer can be left on within the mxd file to show the text information. Another difficulty in the importation is the spatial reference information. The AutoCAD file does have a simple coordinate system. While the survey can be tailored to the coordinate whether as state plane system or UTM, ArcGIS cannot understand what the coordinate within AutoCAD dwg stands for. Therefore, to correct the spatial reference warning, the dwg file would have to be assigning a coordinate system by ArcCatalog. This would have to be done before importing the layers. The importation of the AutoCAD survey file is very useful to compare updated topographic information to, for example, a USGS 7.5 topographic quad or to view historical changes between surveys over the years. The ArcGIS can work with CAD programs if ArcGIS can read their files. The author express that third-party software could make the transition easier for novices but experienced ArcGIS user can quickly import CAD layers into the mxd file for future use. |
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