Puget Sound LIDAR Consortium

1998-99 Snoqualmie LIDAR survey

In 1998 the U.S. Geological Survey contracted for an experimental LIDAR survey of the Snoqualmie valley. After an instrument malfunction that led to reflying a large part of the survey in 1999 and the usual delays in processing, the completed survey was delivered in 2000. With the permission of the USGS, we are posting the gridded DEMs from this survey here.

DEMs are 5-meter grids in UTM10, horizontal datum NAD83, in rectangular patches that enclose all or part of the relevant USGS 7.5-minute quadrangle. Elevations are in floating-point meters and the vertical datum is NAVD88.

These data were considerably sparser than the PSLC surveys (nominal 5 meter spot spacing in X and Y, as opposed to 1.5 meter X-Y spacing for the PSLC surveys). The Fall City quadrangle and the southern part of the Snoqualmie quadrangle were double flown, giving a higher data density.

First-return data are not available. Here you will find bare-earth DEMs and shaded-relief images of the bare-earth DEMs:

download file	contains
<quadname>.e00.zip	ZIPped .e00 file of 5-meter DEM 00Readme.txt
<quadname>.j.zip	ZIPped JPEG of shaded-relief image <quadname>.jgw (world file) 00Readme.txt
<quadname>.jpeg	JPEG of shaded-relief image

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Disclaimer

We have taken considerable care to ensure that these topographic survey data and derived images are as accurate as possible. We believe most of these data are adequate for determination of flood hazards, for geologic mapping, for hydrologic modelling, for determination of slope angles, for modelling of radio-wave transmission, and similar uses with a level of detail appropriate to a horizontal scale of 1:12,000 (1 inch = 1,000 feet) or smaller and vertical accuracy on the order of a foot. Locally, the data are of considerably poorer quality.

Users should carefully determine the place-to-place accuracy and fitness of these data for their particular purposes. For many purposes a site- and use-specific field survey will be necessary.

Note about formats for raster elevation data

We have posted DEMs as zipped .e00 files. The .e00 file format is verbose ASCII, expanding gridded DEMs to approximately 4 times their binary size, but is easily imported into Arcview and Arc-Info and carries projection information. It can also be deciphered relatively easily: see, for example, geocities.com for format description. If this format doesn't work for you, please let us know!

The DEMs and images are in rectangular tiles that include the named 7.5-minute quadrangle. Adjacent tiles overlap along their margins. There should be no invalid values in these DEMs.

Update Policy

As we approve data from the contractor they are posted here. Areas within many of the bare-earth DEMs can be improved by further processing of the all-return point data to better isolate bare-earth points. As we make these improvements we will upgrade the bare-earth DEMs and derivative images posted here. Users of these data may want to return to this web site to check for updates, or register to receive automatic notification of updates for areas they have downloaded.

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