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Surveying GIS

November 1, 2001
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Binge answers the reader question, "Why didn't you include some links to sample data?"

Binge
My goal in the first three articles of this column was to provide interested people in the surveying community with an accurate portrait of what a Geographic Information System is, what it does and who uses it. The question I heard most often from readers was “Why didn’t you include some links to sample data?” And the answer is I wasn’t quite ready to cross that bridge.

The reason I demurred from opening the doors to the data warehouses stems from a recurring theme in the feedback from GIS training sessions. “This is great! But how can I use it with my own data?” Overwhelming new GIS users with the bewildering catalogs of available GIS data is often more of a distraction than a training aid. Thus, my strategy is to encourage readers to look around their floppies, hard drives and networks for some point files, CADD files and even some imagery files.

Now that we have ostensibly been successful in addressing the question of whether or not it could be practical to manage survey data in a GIS format, let’s take a more in-depth look at the world of GIS data.

Data Types

All of the data contained in a GIS falls into two general categories: spatial data and descriptive data. Spatial data has two main subcategories: raster data and vector data.

Raster data is made up of rows and columns for storing images. Groups of cells with the same value represent features. Imagery and scanned material are examples of raster data. Vector data is based on coordinates. It is most often used to display linear features. Every unique linear feature is represented as an ordered set of vertices. Common vector data entities are polygons and arcs.

Descriptive data also has subcategories: attribute data and metadata.

Attribute data can be a text string or numeric value that contains information about a spatial property that is displayed as raster or vector data. This attribute data is linked to the spatial data by containing georeferenced or geocoded fields. Attribute data for the spatial properties is either transferred from legacy systems, added from source data documents or developed from scratch. Metadata (data about data) is an often neglected area. In the past it was typically abstracted from the source data. Now it can be created “on the fly” in the measurement phase using data dictionaries. Source data generally refers to any digital, spatial, tabular or narrative data used to construct the database that produces a GIS map. But it is also often understood to be data that was independently developed for its own purpose. Subsequent to that purpose, and after the fact, it is acquired and converted into GIS data.

FAQ

Q: How does the data get into a GIS?
A: Data capture is considered to be the most costly element of developing and maintaining a GIS. This is consistent with the premise that the field work is usually the most costly part of a surveying project. In the beginning, most GIS layer construction was done by digitizing hard copy maps or by using coordinate geometry routines referenced to a selected datum to convert tabular map unit information to digital data. Scanning came along later. Data conversion of CADD files was and still is a common source. Now with improvements in the accuracy, precision and efficiency of GPS technology and especially the deactivation of Selective Availability, measurement is rapidly becoming the preferred method. Mobile GIS is a new innovation that allows users to work directly on the GIS layers onsite in real-time by linking GPS units directly to the database.

If your map resembles this one, you have successfully completed GIS for Surveyors as described in these first four articles.

Data Categories

There are generally speaking four major categories of GIS data: Land Information Systems, Natural Resource Inventory, Road and Street Networks, and Facilities Management.

Parcel Data is the core of the Land Information System (LIS). It warehouses all of the property characteristics. Parcel data typically contains ownership information, area, value, zoning status, and often more. Parcel data is what drives the commercial aspects of GIS. Most privately owned organizations license this data. Municipal entities vary widely in their approach to handling distribution of their digital data. Some make it available at either a small reproduction cost or free. Others license the data and some maintain the database exclusively for internal purposes. Survey data, both property and control, is part of the Land Information System.

Natural Resource Inventory data includes information about watercourses and flood plains, geology, wetlands, forests, agricultural lands, aquifers, wildlife habitats and recreational resources.

Road and Street Networks are data sets that have many business applications. They are used to route, not only emergency and police vehicles, but a variety of commercial and publicly owned transportation enterprises. Those driving directions you find on the Internet are a good example of the use of GIS road network data.

Facilities Management is a data set that contains information about both underground and overhead utility networks and systems.

Data sets that contain a great deal of information or many fields are described as “rich” data sets.

Data Availability

The amount, type and quality of data available varies around the country. But the quantity of data is immense. I visited over 200 sites to help research this article. Many of these sites have links to the large data warehouses. In this piece we will only visit some of the more well-known sites that warehouse data at the national level.

Datums and Projections

Geographic coordinates (latitudes and longitudes) are a good choice to use to construct a GIS project. A large volume of easily obtainable map data comes in geographic coordinates. Datum is a factor. There is a shift between NAD27 and NAD83 that varies by location. The projection is usually specified in the metadata. UTM (Universal Transverse Mercator) and State Plane Coordinates are also very widely used in GIS mapping. Both of these specify a projection.

ArcData Online (www.esri.com/data/online/index.html) is a data warehouse operated by ESRI (Environmental Research Systems Institute) in cooperation with a variety of partners that provide or license data.

This page, relatively unassuming in appearance, is loaded with data links. The four bullets in the upper center portion of the page describe its available services: create dynamic map images, download free geographic data, license commercial data sets and create custom map reports.

The first and fourth options involve online exercises. Let’s skip those for the time being and begin with the second bullet. Most of the “free geographic data” is in the form of U.S. Census Bureau TIGER (Topologically Integrated Geographic Encoding and Referencing) Files. To download the data you need to register using the online form. Then follow the links to the area (by county) of your choice. If you don’t already have ArcExplorer, you will find a link to download this free data viewer on nearly every page you visit. Choose “Select By County,” which will bring up a list of layers of the area of your choice. Select a data set for your area and download it. Save the .zip file to your hard drive. All of this data is in geographic coordinates.

Now let’s visit the third bullet, “license commercial data sets.” The choices here are U.S Street Data by DynaMap, USGS Topo Maps by Horizons Technology and FEMA Flood Data.

Dynamap allows you to download street and feature data by zip code. The price is $15 per area with some price breaks for multiple data sets. Topo maps from Horizons are priced at “about $15 per quad.” They are available in the projection and datum of your choice. There are also large area data sets. FEMA Flood Data is available for about 1,200 U.S. counties at $35 per county.

It would be easy to spend an entire day following links from this site alone, but we need to move on. If you do have some time, two other useful pages on the ESRI website are Virtual Campus (http://campus.esri.com/) and GIS For Your Specialty (www.esri.com/industries/index.html). Virtual Campus is one of those places that offers some of the online GIS education we talked about last time. GIS For Your Specialty has a page for surveying you might find interesting.

The link http://www.ngs.noaa.gov/products_services.shtml takes us to the National Geodetic Survey Products and Services page. Under the Data Column click “Data Sheets.” You have several search options. “NGSMap” is the newest. I find the Rectangular Search and USGS quad usually best suit my needs. Click the USGS quad link. Then click on the USGS Mapfinder link, click on Conterminous U.S. Map and “drill down” until you find a quad or two that falls within your area.

Identify a quad, then return to the search page. Type the name of the quad. Often several candidates appear. Select the correct quad “Data Type Desired” and click “Submit.” A list of points should appear in your search window. Now you have more choices. First “Select All.” “Get Data Sheets” will produce a voluminous file with the entire NGS history of the points in your search screen. For map making, “Move” is a better choice. Move directs you to save to a file. Save your points to disk as a text (.txt) file. This file will contain latitude and longitude in degrees, minutes and seconds and usually an elevation along with other descriptor fields.

While you’re on the NGS web pages, return to “Products and Services.” Click “Software” then “Geodetic PC Software” and select “CORPSCON.” Follow the links and save it to your hard drive. You will find this a useful utility to convert coordinate data between systems. It is free, easy to install and very powerful.

Now we need to do a little editing. It is always a good idea to make a backup copy of the original files before we start editing them. All we really need to display the points are x and y values and a description. MS Access or Excel are good choices, but any text editor will do the job. Strip out the “N” and “W” in front of the latitude and longitude values and insert a decimal point or a space between the degrees, minutes and seconds. Depending on the size of your data set, this could take a while. You may also need to add a minus sign in front of the latitude.

If you have ArcView, it is easy from here. You simply add the points as an “Event Theme” just like we did in the last exercise. Remember, we are working in geographic coordinates. ArcView reads geographic coordinates in decimal degrees. If you have ArcExplorer, you will need to convert your text file to either a shape file or a .dxf. Any CADD or COGO program that imports points and writes out to .dxf will do.

Unzip the TIGER Files and enter the themes into your viewer. Adjust the draw order and modify “Theme Properties” in ArcExplorer or “Legend Editor” in ArcView to get adequate contrast between the themes in the map view. Then add your point file. If it is in the form of a shape file, make it current and set the label property to the description field. What we have now done is constructed a GIS basemap showing geodetic survey control points for your area using free data. If you have a map that looks something like the figure on page 64, give yourself an “A.” You have successfully completed the crash course in GIS For Surveyors embedded in these first four articles. If you want to explore more data options, try www.gisdatadepot.com. This site bills itself as containing the largest volume of free GIS data on the web.

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