YCT Multi-state Assessment 



February 1 0, 2003 



Appendix C - GIS scale issues 



Differences between 1 : 100,000 scale stream layer and 

 1 :24,000 scale stream layer stream lengths 



Wendi Urie 



Gallatin National Forest 



Bozeman, Montana 



December 2001 



We conducted a comparison of stream lengths on the 1 : 100,000 scale LLID layer and the 

 1 :24,000 scale National Elevation Dataset available for portions of the study area. We selected 

 1 streams each from the Jefferson, South Fork Flathead and Lower North Fork Clearwater 

 watersheds for both scales. Lengths were compared using regression analysis. The 1 : 100,000 

 scale streams were found to be only 1% shorter than their equivalent 1 :24,000 scale 

 representation. Thus the streams included in the 1 : 1 00,000 scale LLID stream layer represent 

 approximately the same number of stream miles as the corresponding streams in the 1 :24,000 

 scale National Elevation Dataset. 



Spatial Variability in Streams Represented in the LLID Streams Layer 



The LLID stream layer contained some variability in the selection of streams represented across 

 the study area. The streams represented in Idaho and Montana (with a few exceptions in MT) are 

 only the named streams. All unnamed streams were not included in the LLID layer. In 

 Washington, Oregon and those watersheds spanning the boarder with Idaho unnamed streams 

 were included. Thus the density of stream miles in a watershed was much greater in these 

 watersheds. To compare two watersheds we looked at the Priest (on the border between ID and 

 WA) and the Upper Coeur d'Alene (ID). There are approximately 1.86 miles of stream per 1000 

 acres in the Priest watershed and 1 .20 miles of stream per 1000 acres in the Upper Coeur d'Alene 

 watershed. There is approximately 35% more stream miles represented in the Priest watershed 

 and thus 35% more potential habitat to include in historic or current range. Therefore the 

 watersheds with unnamed streams represented had the potential for approximately 35% more 

 historically and currently occupied range. The following tables list the miles of unnamed 

 streams in these watersheds that were included in the historically and currently occupied habitat. 



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