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That form of axiomatic logic doesn't only apply to sports. It's true of any profession, including surveying. Indeed, the Fundamentals of (Land) Surveying is the title of the initial phase of the Professional Surveyor examination process. Now, GIS (Geographic Information System) has even made its way into the surveying examination process. In this column we are going to revisit some GIS basics for the benefit of those who may need to work on their GIS fundamentals.
So, Where Do We Start?Excellent question! Any GIS project or application begins with the same thing a survey starts with: a needs assessment. Let's address a few fundamental questions:
- What do I want to use my GIS for?
- What is the extent of the geographic area of interest?
- What kind of data do I have?
- What kind of data do I need?
- What is the best method of obtaining it?
- What kind of software do I have?
- What kind of software (if any) do I need?
The good folks at Autodesk say there are three distinct activities associated with digital data: creating, managing and sharing. This installment will address the rudiments of creating and sharing. Managing the data is a broad subject that will be addressed in the future.
Creating the Base MapIt's always a good first step to start with creating a base map for your GIS. A base map is a layer, or more often, a group of layers, that depicts an outline of your area of interest and includes some core function attribute data in tables. Fortunately most of our world (and yes, even parts of others) has already been mapped. And even better, a good deal of map data is readily available, much of it free and downloadable from the Internet.
I have located a site that has some free base map data available for my location (see Figure 1) and downloaded some Shape files. Now I want to view these layers so I can develop a design for my GIS database. To proceed further, I need to make some decisions about the software I plan to use. What do I have? And what do I need? Napoleon Bonaparte allegedly said, "Fight with the weapons at hand." Since AutoCAD (2005), ESRI ArcView 9.0 and Microsoft Office are available to me, I will use those for the purposes of this exercise.
OK, now how do we add our existing CAD and point files to our new GIS project? Well, first we need to make sure all of our existing files are in the same projection and datum as our base layers. In this case, the metadata tells us the base map files are in California State Plane Coordinate Zone VI. And the units are feet. Yes, ArcMap projects data on-the-fly to conform to the designated base system if the data set is recognizable or has a projection (.prj) file. ArcMap fixes the first data set as the base layer and then analyzes all subsequent layers to match them spatially to that base data set. For database use, keeping the system and units as uniform as possible works best.
The CAD files enter the Map View much the same as Shape files, except you double click on the .dwg file to display the usable layers. Select those you want from the display dialog and click "Add." Points can be entered into a GIS from any standard tabular format that contains usable X,Y fields. From the "Tools" menu in ArcMap, select "Add X Y Data." Navigate to the point file. Then arrow down to the "X Field" and "Y Field" boxes, and once again click the "Add" button. See Figure 2.
We have now created a rough, rudimentary framework GIS base map project file. It isn't sophisticated, but it contains all the basic required elements.
Exporting GIS DataIn our base map project we were able to enter our CAD files. We can view them along with any of the other base map files, and query the data for reports or analysis. But what if we wanted to "share," as in export, some GIS data from our project and use it as a framework for a CAD project?
In our sample base map project we have a Public Land Survey System (PLSS) layer. We have some survey control points as well. Now we want to export some of them from our GIS project and import them into a new CAD project drawing.
Now, to export the linework, click on the ArcCatalog icon. (Give yourself a gold star if you remembered that the Conversion Tools for ArcGIS 8 are in ArcCatalog in ArcGIS 9. For more information on this, see "What's New in ArcGIS 9?" Surveying GIS, POB, February 2005.) In the ArcCatalog "Conversion Tools" menu, choose "Shapefile to DXF." In the pop-up dialog box, navigate to the Shape file you exported from ArcMap in the "Input Shape File" box. Then type in a name for your .dxf file in the "Output DXF File" box and click "OK." See Figure 6.
It's a good idea to open the DXF in AutoCAD and check the results. Then you can save it as a .dwg file.
Getting The PointsHow do we move points from GIS software to CAD software? There are two ways. In Autodesk Map we could import a Shape file with the points. But first we need to understand another fundamental. And that is the fundamental difference between a CAD product and a GIS product. The CAD product opens in the "Edit" mode (in model space). It assumes the user wishes to add or edit data. A GIS product does not open in this mode; the user selects his mode. In GIS products, the primary use tends to lean more toward analysis than data editing.
If we import the point Shape file to CAD using Autodesk Map, we will be in a read-only mode. To change to an editable mode, we need to convert the point positions to CAD data. This is easy to do using Microsoft Excel because the .dbf file in a Shape file contains all the point attributes. But our points came in via the table route, so we need to execute a few additional steps.
ArcMap will display this type of layer as an "event" in the Table of Contents. But the "event" point file doesn't appear in the list of selectable layers. So we need to change it to project data. In the Table of Contents, right click on the point "Events" layer. Choose "Data, Export Data." Choose the "Use same Coordinate System as the data frame" option. You also have the choice of making your point layer a Shape file or a personal geodatabase.
Open the new .dbf file of exported points in Excel. Delete all cells except point ID number, northing, easting, elevation and description. Export the points as a .txt file. Edit into a standard comma delimited PNEZ format (see Figure 7) and use the "Import" points command in AutoCAD to bring in the point files. We now have our database record point and linework for Section 8 placed in a CAD file framed and ready for our survey work. See Figure 8.
This exercise demonstrates how to successfully create and share GIS data. In my next column I will discuss the broader issues of how to manage data within a GIS.