A 1.127Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE
This article explains how to use GIS to obtain the latest NGS control point datasheet with just the click of a button.
The National Geodetic Survey control point database contains survey control information, such as Latitude, Longitude, Height and Gravity Data for published control points. The NGS provides on-line access to these control data, and offers a variety of download formats (see Figure 1). For compatibility with GIS, one of those download options is the ESRI ArcView shapefile format, which most GIS software can read. The shapefile format provides a graphic representation (point) of the record (which one may display on a map) along with tabular information. The tabular information is an abbreviated subset of the full NGS datasheet record (see Figure 2). These tabular data are very useful for searching for points based on certain criteria, such as monument stability, or vertical order, and for symbolizing the points such as showing vertical control versus horizontal control. The best field of all the tabular data is the DATA_SRCE field that contains the path name to the NGS data on the NGS server. This is very significant because this path name is a hyperlink to the NGS datasheet, which means that instead of downloading a set of datasheets, which would have to be updated periodically, one can get the latest datasheet any time by simply clicking on the point in the GIS map.
The shapefile format is a small file size and easy to download compared to downloading a set of datasheets. Additionally, the shapefile, when used in GIS software, allows the user to make any map of his/her choosing, overlay the NGS control with other GIS data (such as local control or aerial photography), and allows for custom searches and queries of the NGS points from within the GIS environment. To take advantage of this great tool one must use GIS software that supports hyperlinks in shapefiles. Many GIS software packages do this including at least one free GIS data-browser that does this.
The process to set this up is very simple. Go to the NGS website (www.ngs.noaa.gov), then go to the datasheet page (Figure 3) and select the ShapeFiles retrieval option. This opens the ShapeFiles Retrieval Page, which offers a few options for searching points in the NGS database. After the doing the search, select the desired points, then download the shapefile and save it to the local hard drive. Once the shapefile of points is stored locally, then fire up the GIS software and load the points into the application. Figure 4 shows the NGS points and the attributes for a point as displayed in a GIS. As mentioned earlier the attributes are associated with the point feature and are a subset of the full datasheet record. See the NGS Explain ShapeFiles page for an explanation of the contents and format of the NGS shapefiles.
Next, set up the DATA_SRCE field as the hyperlink field. Each software does this a little differently; depending on the particular GIS software used, the DATA_SRCE field might need to be renamed as something else. In this example, Figure 5 shows how to set the DATA_SRCE field as a hyperlink in ESRI’s ArcMap. Here it is simply a matter of indicating which field to use as a hyperlink and specifying that the hyperlink represents a document.
After the hyperlink is set up, clicking on a point will retrieve the NGS datasheet from the NGS server. Figure 6 shows the shapefile of NGS points overlaid on local aerial photography in a GIS map. The information for a point shows in the table information window, and the hyperlink shows on the map, as well as in the table (which may be difficult to read in this graphic). With the ArcMap hyperlink tool, a single click on the point sends a request to the NGS server to retrieve the datasheet for that point and then displays that datasheet in a web browser window (Figure 7). That datasheet may then be printed, or saved locally using the web browser file save option.
The shapefile option for NGS control point retrieval is fairly simple to use, yet extremely powerful. By using this option, the surveyor takes control of the NGS data on the desktop, yet has instant access to the latest datasheet information for each NGS point.
Rj Zimmer is registered in Oregon and Montana, and has more than 25 years of surveying experience in the private and public sectors. He is the GIS Consultant & GIS Center Manager for the City of Helena Lewis & Clark County Geographic Information Services Center in Montana.
A 1.127Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE