Hydrography

Map Reading 101 says that contour lines should come to a “V” where they cross a stream, and the “V” points in the up-stream direction. Right?

WRONG.

We are entering an era now where high resolution terrain data, complete with 1ft or 2ft contours over large areas, are being created and delivered without any hydrography. That means that much of the new topographic data being produced today (digital elevation models and contours) is being delivered without any stream data at all. So what happens when you line up the “old” stream lines (typically 1:24,000 scale) against the “new” high resolution topo? Don’t be surprised at the results.

Hydrography data are the “blue lines” on a topo map. They are either polygons (ponds/lakes/rivers) or lines (perennial streams/intermittent streams). At the national level, this data is maintained as part of the USGS sponsored National Hydrography Dataset (NHD). The current USGS standard for LiDAR projects however, is to deliver topographic data (digital elevation models) that have been “hydroflattened” with the use of breaklines, but only for “larger” waterbodies more than 100 ft wide. These LiDAR projects do not include any hydrography as a standard deliverable.

The good news? The USGS is now looking much more closely at aligning their elevation and hydrography programs so that more consistent updates are on the way.

Meanwhile, at GroundPoint hydrography updates are standard. We use current NHD as the basis for ensuring that all hydrography, not just “LiDAR breaklines”, are included in your projects. And any additional streams previous not mapped but clearly evident in the data (imagery and/or LiDAR) get included as well. We use a standard process to provide you with consistent stream definitions and stream density across your project area. Meaning that when you look at your new contours, they come to a “V” when they cross a stream. Just like they are supposed to.