Cowlitz County was concerned for possible errors in topography and subsequent misinterpretation of elevation data as a result of heavy evergreen tree cover, common in Cowlitz County. For this reason all Cowlitz County contracted projects required an "obscure" boundary to be digitized around such trouble areas. Cowlitz County suggests such, "obscure areas" be viewed/plotted simultaneously with any processed DTM data to aid in interpreting the DTM model (tm_elev values) accurately. The accuracy of this boundary has not been thoroughly checked other than that of visually comparing a plot of "obscure" boundary on a matching ortho photo and photo identifying the density of the tree cover. When such a comparison was made the accuracy of the "obscurity" seemed to be accurate and complete.
For Department of Natural Resources (DNR)10 meter DEM Data:
Ten meter DEM data was developed by the Department of Natural Resources
(DNR) in cooperation with the United States Geologic Survey (USGS) with
the sole intent to match the USGS published 7 ½ minute quadrangles
contours and to improve the accuracy of the existing 30 meter DEM USGS
data. Cowlitz County relies on the accuracy report of DNR concerning its
product and intended use and has conducted no further tests of its own.
Exerts from DNR’s meta data report are:
Tests conducted by DNR have shown that generation of contours from the 10-meter DEM data reproduces the published quadrangle contours extremely well, preserving even the very small quad contour details. This (DNR new 10 meter DEM product) is a tremendous improvement over the previously existing USGS 30m DEM data which, in a majority of southwest Washington , was derived from orthophoto profile data that had not been edge matched and was unrelated to the published contours. Generation of contours from the previously available data produced several anomalies including "seam" effects on quad edges, "banding" due to the profiling methods used to collect the data, unrealistic artifacts in stream edges, river channels, and a poor and often unrecognizable relationship to the quad contours.
The new 10-meter DEM data is in essence, a digital representation of the published USGS 7.5' Quadrangle contours and is suitable for use in any instance where the published 1:24000 Scale USGS Quadrangle maps had previously been employed. The data is not intended for engineering applications which require higher accuracies than that of the published 7.5' Quadrangles.
Non contracted projects, (acquired) meet the following specifications :
10 meter DEM (40') Foot horizontal accuracy
1:600 map scale (1/2') vertical accuracy
1:1200 (1') vertical accuracy
1:2400 (2') vertical accuracy
10 meter DEM (10') vertical accuracy
Final deliverables to Cowlitz County consisted generally of DTM data only. However contracts with Cascade Mapping, and the Army Corp of Engineers produced also finished contour lines at 1 foot (Cascade project) and 4' (Army Corp of Engineers).
For Department of Natural Resources (Wa.) DEM Data.
In late 1996, Washington State Department of Natural Resources (WA.
DNR) entered into a workshare /costshare agreement with USGS for the production
of Level 2, 10 meter-DEM data to provide DNR GIS users with elevation data
that accurately represents the contours of the published USGS 7.5' Quadrangle
maps.
The DEM production itself was performed by USGS and consisted of scanning the USGS 7.5' Quadrangle hysography separate, converting the raster contours to vectors, "tagging" the vector contours, and subsequently extracting gridded elevation data on a 10m x 10m spacing. The individual quad vector contours were edge matched before DEM extraction to eliminate "edge" and "seam" effects.
The individual quad DEM’s have been projected into Washington State Plane coordinates (from UTM) in NAD 27 South zone and appended into one large ARC Info coverage. The DEM data has been converted to NAD 83-91 by Cowlitz County.