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3D Topographic Contour Lines

       

For many of our customers, contour lines are a very important layer. Over the years, the quality of our contour layers has improved tremendously. Now, in many areas they are equal in quality to the contours from the paper maps, and have the advantage of being in CAD format as labeled, 3D polylines.

 In general, there are two quite different sources of contour lines. The first are contours mathematically derived from a Digital Elevation Model (DEM, or DTM). The second are contour lines that have been digitized from a paper source, and are called Digital Line Graph (DLG) contours. These two sources have their advantages and disadvantages:

DEM Contours	DLG Contours
Selectable contour intervals Selectable vertical datum Approximation of original paper map contour lines Tend to "wander" in flat areas 3D Polylines Elevation labels Corresponding 3D XYZ surface available	Fixed contour intervals Fixed vertical datum Follows original paper map contour lines precisely Follows paper map, even in flat areas. 3D Polylines Elevation labels No XYZ 3D surface

TopoDepot Contour Layers:

1:24,000 Contours from 10 Meter Digital Elevation Models

Sylvan Ascent's 24K 10 meter DEM contour layer is generated from the USGS 10-meter Digital Elevation Model DEMs (or more precisely, from the USGS 1/3 arc second National Elevation Database (NED)). Contours generated from this data follow the contours on the paper maps very closely.

The 10 Meter DEM data consists of a grid of elevation values posted every 1/3 arc second, or approximately every 10 meters. We run this grid through a 3D contouring program to generate the contours. We have found this produces excellent results in most areas. Using this approach also allows you to specify the contour interval. We recommend you use the same interval used on the USGS 7.5 minute paper quad, but it is certainly possible to use any interval. 

Click here for more information on the 10 Meter DEM Contours

The 10 meter DEM data is now  available in all areas.

For USGS information: http://seamless.usgs.gov/website/seamless/products/3arc.asp

 

1:24,000 Contours from 30 Meter Digital Elevation Models

We also have data from the USGS 30 Meter DEM, which is similar to the 10 meter, but coarser, with postings approximately every 30 meters (every 1 arc second). The USGS has 100 percent coverage of the USA with this data.

TopoDepot also allows you to specify the contour interval with this data, and generate 3D surfaces. While not as good as the 10 meter data, contours generated from this data follow the contours on the paper maps closely.

For USGS information: http://seamless.usgs.gov/website/seamless/products/1arc.asp

 

1:24,000 DLG Contours

The USGS has released a large number of 24K DLG data files. The lines are 3D polylines in AutoCAD DWG and DXF format. The quality of this data is extraordinary, and follow the original paper map contours very precisely, since they were digitized by the USGS from the original 1:24,000 paper maps.

The 24k DLG data is not yet available in all areas. Click here to show a status map.

Accuracy: Map accuracy for these maps corresponds to National Map Accuracy Standards for the original data from the US Government. The DLG files have a stated horizontal accuracy of approximately ±33 feet.
 

For USGS Information: http://rockyweb.cr.usgs.gov/nmpstds/dlgstds.html

1:100,000 DLG Contours

The USGS released the 100K hypsography (contour line) series in 1986, contour lines for the continental USA. The lines are 3D polylines in AutoCAD DWG and DXF format. The quality of this data appears to be good, but will not be the same as 24K contours.

Contour Intervals: Since the source material was the 100K metric quadrangle series, elevations were originally expressed in meters. We have converted elevations to feet. This results in somewhat strange contour intervals since the even meter intervals are converted to feet.

Accuracy: Map accuracy for these maps corresponds to National Map Accuracy Standards for the original data from the US Government. The DLG files have a stated horizontal accuracy of approximately ±166 feet.

USGS Information: http://rockyweb.cr.usgs.gov/nmpstds/dlgstds.html

 

The TopoDepot Decision Tree for determining the default selection in TopoDepot is:

	If 24K DLG contours exists, use them.
	else if Level 2 USGS DEM data exists, use this.
	else if 100K DLG contours exist, use them.
	else if Level 1 USGS DEM data exists use this.