Point of Beginning

Surveying GIS

August 1, 2005

Figure 1. The San Diego Association of Governments Data Warehouse website offers free base map data.
One of the classic clichés in the world of sports is "We need to get back to fundamentals." This is most often heard from coaches when a player or team is struggling. What these coaches are talking about is re-focusing on the core skills that brought those athletes into their professions in the first place, and then reinforcing them.

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?
After we examine these fundamental questions, we would do well to address another more ticklish fundamental question: Who is going to use our data? For the purpose of this exercise, let's assume we are going to use our GIS to help manage our own survey projects.

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.

Figure 2. Adding point files into a GIS.

Creating the Base Map

It'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.

Figure 3. Query to find the record of interest.
First, we will launch ArcMap and add the downloaded files to a GIS Project file. Project files are designated .mxd in ArcMap. Click the "+" symbol and navigate to the directory where the Shape files (in this case) reside. Select the files and click "Add."

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.

Figure 4. Record selection in Map View.

Exporting GIS Data

In 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.

Figure 5. Map View with selection menu shown.
We have two ways of locating our area of interest. We can zoom in if we know where to look. Or we can query the data in the appropriate table. In the "Data Frame," right click on the PLSS layer. My area of interest is known to be Section 8, Township 11S, Range 6E. A simple query finds the record and highlights it. See Figure 3. Select the record by clicking on the column and then close the table. The selected record is now highlighted in the map view. Use the "Zoom" tool and make a box around it. See Figure 4=. Now from the "Selection" menu make the "Selectable Layers" active by clicking on it in the menu and checking the boxes in the pop-up dialog. See Figure 5. Then, using the "Select Features" tool (the white arrowhead on the main toolbar), hold the left mouse button down and draw a box around your area of interest. Or simply click in the center area of the polygon. (It is now highlighted.)

Figure 6. A .dxf file is named in the "Output DXF File" box.
In the Table of Contents, right click on the layer you made the selection from. Choose "Data, Export Data." Name the file or accept the default. Click "OK."

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.

Figure 7. Standard PNEZ comma delimited point file.

Getting The Points

How 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.

Figure 8. CAD view displaying imported data view.
For this exercise, let's use Shape file. Say yes to the "Add data to the View?" prompt. Your new point Shape file will now appear in the list of "Selectable Layers."

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.