I am a great fan of the Northwest USA map created by John M, and have been using it for several months now. I have noticed that a chunk of Idaho is missing. There is no topographic data for the area between 43-44 degrees latitude and 113-114 degrees longitude shown either in MapSource or on my gps device. Do others show the same gap?
I did notice errors when I installed the map with MapSetToolkit. Is that the source of the problem? Is there an easy fix, or is the data just not available?
David
I would look into -OZ-'s Idaho TOPO if I were you:
http://www.gpsfiledepot.com/maps/view/45/
Should be gap free.
Thanks. I do use the Idaho TOPO map also, but since it has only half the vertical resolution of the Nolrthwest USA map, I prefer the later most of the time. When navigating cross country and trying to select the best route, the extra resolution sure helps.
Numbers do not mean everything. Simply having lines closer together does not make for greater resolution in terms of a more detailed (finer resolution) description of the landforms/surface. We make the contour lines from an elevation grid (NED/DEM) we download from USGS. USGS constructs most of these grids using various methods from digitized contour lines from the 7 1/2 minute (1:24,000) printed topo quads. I would estimate about 25% of the ID source quads use a 20ft contour interval. Because much of ID is highly sloping, the contour interval used in these areas is 40 or even 80 feet. A CI of 20 is simply constructing 1 or 3 lines between known lines - what the land looks like inbetween is nothing but a guess. OZ used a dual CI for ID to best describe the moutainous areas and the flatter 'plains'. I also like the visual crutch of closer lines, but with my academic and professional background I am well aware of what is likely to be true and where to be cautious. The NW USA is even more generalized as the author mentioned he used a grid with 30m spacing between between 'control' points - 'everyone' else is using the 10m data which has nine times as many 'control' points to describe the detail of the land surface. If NW USA suites your needs, use it; but do not be fooled by the CI.
I hadn't thought about the source data, but what you say about interpolation makes a lot of sense. The contours can't be any more accurate than the source data. Thanks!