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Converting Legacy Data to GIS Database Format With AlignDB

The most expensive portion of any GIS implementation has traditionally been the conversion of legacy data into a GIS Database format. Although the AlignDB product was designed for automating the creation of Alignment sheets from a CAD base map, it has also become a productivity tool for the conversion of legacy CAD data to a GIS database format. Statistically over 95% of legacy binary data is contained in existing CAD documents and images.

Utilizing the AlignDB product, the extraction of intelligent spatial information from these files is both quick as well as cost effective. The AlignDB product allows the user to quickly classify existing features within the drawings as feature survey points, feature symbols or feature vectors. Once classified, the user can then begin to quickly scan through these files extracting the data to an external spatial database. This database sorts the data by the user classification and calculates station locations relative to any user-defined alignment.

All of the scanning functions, averaging around one minute per drawing can handle up to 10,000 drawings in an unattended mode in 7 days or in 21 workdays if attended, provided the data is relatively consistent. This document details the steps necessary for completion of this procedure.

The first step in converting CAD documents into data is determining if the drawing is Geo-referenced, and if so what projection was used. If the drawing is located anywhere close to 0,0 then it is pretty safe to say the document is not on a valid projection. If this is the case, the first step is locating this document and associated documents in a valid projection. Several methods are possible depending upon the accuracy needed.

The first less accurate, but inexpensive method is to download digital USGS quads of the known area, then move the drawing into the correct location. Using this technique will allow only the very basic of x, y locations to be extracted with the accuracy determined by the user and the existing quads.

The second inexpensive method, but more accurate is to have a surveyor GPS locate major features along the route. These GPS points can now be imported into a drawing and the old drawing can be moved into place to match these surveyed points. If base station correction is used with the GPS survey, then sub-meter accuracy will be attained at each of the surveyed features. If no base station is used, then at best three-meter accuracy will be attained at the surveyed points.

This document will attempt to describe a method of locating existing data into a known projection using GPS surveyed features. Because an exact match to every legacy data situation cannot possibly be made, the techniques described should be adapted to best fit the user's needs.

Before beginning the conversion of the data, a little research must be done into the existing CAD documents. First, review a sample document to determine what layers need to be defined as feature crossing layers. Second, review the sample document to determine what symbols need to be defined within AlignDB so that they may be extracted with proper descriptions.

In understanding which layers need to be defined as crossing layers, the user must determine which layers contain CAD elements that represent features that should be stored in the database, for example, river crossings, road crossings and other similar features. This may be done in the Add Layer Dialog.

Another option is to select the Auto-Add Crossing Layers, which will add all Layers in the current drawing as crossing layers.

Then the user should remove those layers from the layer table which contain CAD elements that do not represent features, for example, dimension lines, leaders and construction lines.

In understanding which blocks to add to the user point table, the CAD document needs to be reviewed for features the user determines should be in the external database. Once determined the user can select the appropriate blocks by either the name in the drop-down list of blocks contained in the current file or by clicking the Select button and selecting the required block.

Once the above steps have been done, the user needs to import the GPS surveyed points into a new CAD document, then Xreference this new file into the existing CAD document. The existing CAD document can now be corrected to fit the newly surveyed points.

Once this is done the user can now define the alignment, and then begin the scanning functions.

To define the alignment, there must first be a polyline entity in the current CAD document that represents the alignment. The alignment geometry will be defined by selecting this polyline. In the New Alignment Dialog, the alignment must be given a name, a description, a beginning station and a default material.

After providing this information and clicking the OK button the user will be prompted to select the Polyline. The alignment has now been defined in the database.

The final steps are scanning the file to tie all the features to the newly defined alignment. These options may be found under the utility dialog.

 

 

First, Scan the file for Crossings. This function will follow the alignment searching for entities that intersect the alignment and that are located on one of the layers the user added to the Layer table as a Crossing layer.

Next, Scan the file for User points. This function will search the CAD document for each of the block features the user defined as a user block and store the location in the database.

The Project database should now contain references for all of the User defined blocks in the CAD document, as well as all objects intersecting the alignment.