A second source of error is the error associated with the GIS layers used in making the delineations. The main layers that were used and an estimate of the horizontal accuracy for each layer is listed.
Counties (+/- 50 meters); see note 1 DEM (30 meter resolution, +/- 15); see note 2 DEM (90 meter resolution, +/-100); see note 2 Geology (+/- 700); see note 3 Hydrography (+/- 50); see note 2 Hydrologic units (+/- 700); see note 4 Land Cover (+/- 250); derived from Thematic mapper dataset; see note 3 Thematic Mapper (+/- 200); original dataset was not terrain corrected and was resampled up to 100m resoultion; see note 3Note 1: Accuracy estimate based on the assumption of a 0.5 mm digitizing error.
Note 2. Accuracy estimate from the "Geo-Positioning Selection Guide" developed by the U.S. Department of Interior, Bureau of Land Management (U.S. Government Printing Office: 1992-676-687).
Note 3. Accuracy estimate based on a comparison of the positions of large lakes and reservoirs and major rivers in the GIS layer with the positions of the corresponding features in the USGS 1:100,000 hydrography DLG coverage.
Note 4. Accuracy estimate based on a comparison of the line along the continental divide in the Wind River Range with the corresponding lines in the counties coverage, which forms the boundary between Sublette and Fremont counties.
Some of the arcs in the Landtype Association coverage were taken directly from the source layers, so the errors associated with these arcs carried over to the Landtype Association coverage. Other arcs were digitized on-screen with a variety of background displays. In this case, there was not only the error associated with the GIS layer used for the background display, but also a digitizing error. The most common background display was shaded relief derived from 30 m DEM data, with the display zoomed to a scale of about 1:24,000. When there were distinct topographic features to follow, the lines drawn were within 1 to 2 pixels of the desired location (30-60 m with 30 m DEM data). When the features were less distinct, the position of the line involved more judgement. Often, selected geology polygons were displayed on top of the shaded relief with hatched patterns to guide the digitizing. In this situation, the horizontal accuracy could vary significantly depending on whether the delineation closely followed the relief or the geology.
Since the accuracy associated with a given delineation depends on many factors and since these factors may vary for different parts of the delineation, it is difficult to specify a single number that reflects the true accuracy situation. Lines based on distinct features visible in the 30 m shaded relief displays are going to be more accurate than lines based on 90 m data or lines based on the geology coverage or the hydrologic units coverage. In an effort to document the source/criteria for each delineation and to provide some sense of accuracy, the arcs in the Landtype Association coverage were attributed with a 3-digit code in the SOURCE field. Codes ranging from 101 to 199 indicate lines obtained directly from other coverages, while codes from 201 to 299 indicate lines digitized on-screen with the specified background display. In addition to the SOURCE field, each arc has a NOTE field that contains a brief description of the criteria used in making the delineation.
All meaningful attributes are included in the polygon attribute table And arc attribute table (.PAT and .AAT) that accompanies the ARC/INFO coverage.
In the polygon attribute table, all coding was done within the fields MCODE, PCODE, SCODE, SSCODE, LTACODE and a descriptive character field, LTANAME.
MCODE: this field is either blank of contains an M to indicate mountains.
PCODE: This field contains a three digit code to indicate the province.
SCODE: This field contains a single uppercase letter (A, B, C, . . .)
to indicate the section.
SSCODE: This field contains two lowercase letters to indicate the subsection.
The first letter can be any letter of the alphabet. The second letter will
be either a 'w' for Wyoming to indicate delineations by Reiners and Thurston
that significantly differ from Freeouf (1996) or an 'f' for Freeouf
where there is relative agreement.
LTACODE: This field contains a two digit code to indicate the landtype association.
Code Landtype Association
11 Mountain (not subdivided into LTAs)
21 Isolated Mountain
22 Granite Hills
25 Channeled Hills
26 Footslopes
33 Dissected Plateau
35 Breaks
36 Mesa
41 Recessional Escarpment
51 Multiple Cuesta and Valley Complex
52 Single Cuesta
61 Glacial Till and Outwash Hills
71 Rolling Plains
72 Rolling Plains and Alluvial Valleys
82 Major River Valley
83 Lake/Reservoir
LTANAME: This field contains the name of the landtype association.
For the arc attribute table, all the coding is in the TYPE, SOURCE and NOTE fields.
TYPE: This field contains a one or two digit code that indicates the line type.
Code Line Type 2 Division 3 Province 4 Section 5 Subsection 6 Landtype Association 11 County Boundary 12 State BoundarySOURCE: this field contains a three digit code that indicates the source GIS layer for the arc, the background display used for on-screen digitizing, or the source map for on-tablet digitizing.
Arcs copied directly from specified coverages
101 County boundaries 105 Hydrologic units 106 Geology 107 HydrographyArcs digitized on-screen with the specified background displays and manually smoothed.
202 Geology
204 Streams
205 30 m shaded relief
206 90 m shaded relief
207 30 m shaded relief with hatched geology following relief using
geology as a guide
208 30 m shaded relief with hatched geology following geology primarily
209 90 m shaded relief with hatched geology following geology primarily
215 No background
216 100 m thematic mapper imagery (4,5,3 (RGB))
218 30 m shaded relief with elevation contour following relief using
elevation as a guide
224 90 m shaded relief with streams
226 30 m shaded relief with slope contour at about 4 degrees following
relief using contour as a guide
227 30 m shaded relief guided by 60 m shaded relief calculated from 90 m > DEM data reclassified into 50 m intervals
229 30 m shaded relief following relief guided by hatched geology and
slope contour at about 4 degrees
230 30 m shaded relief following relief guided by hatched geology and
slope contour at about 10 degrees
231 30 m shaded relief with slope contour at about 10 degrees following
relief using contour as a guide
Arcs digitized from paper maps
301 Lusk (1:100,000, folded) 304 Torrington (1:100,000, folded) 305 Douglas (1:100,000, folded) 306 Laramie (1:100,000, unfolded) 307 Rock River (1:100,000, unfolded) 308 Torrington (1:100,000, unfolded) 309 Baggs (1:100,000, folded) 310 Rawlins (1:100,000, folded) 311 Cheyenne (1:100,000, unfolded) 312 Chugwater (1:100,000, unfolded)NOTE This field contains comments concerning the source of the arc or the criteria used when digitizing the arc.