Welcome, Guest. Please login or register.

Login with username, password and session length
Forums Search:  


Author Topic: Northwest USA Map question  (Read 3425 times)

astroman

  • Jr. Member
  • **
  • Posts: 3
  • Karma: 0
    • View Profile
Northwest USA Map question
« on: August 18, 2009, 02:52:18 PM »
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

Indrid Cold

  • Moderator
  • Expert Advisor
  • *****
  • Posts: 919
  • Karma: 20
    • View Profile
Re: Northwest USA Map question
« Reply #1 on: August 19, 2009, 08:24:33 PM »
I would look into -OZ-'s Idaho TOPO if I were you:
http://www.gpsfiledepot.com/maps/view/45/

Should be gap free.

astroman

  • Jr. Member
  • **
  • Posts: 3
  • Karma: 0
    • View Profile
Re: Northwest USA Map question
« Reply #2 on: August 20, 2009, 04:38:29 AM »
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.

maps4gps

  • Expert Advisor
  • *****
  • Posts: 1524
  • Karma: 17
    • View Profile
Re: Northwest USA Map question
« Reply #3 on: August 20, 2009, 06:52:00 AM »
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.

astroman

  • Jr. Member
  • **
  • Posts: 3
  • Karma: 0
    • View Profile
Re: Northwest USA Map question
« Reply #4 on: August 20, 2009, 09:14:30 AM »
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!