The ISOM 2000 says:

102 Index contour: Every fifth contour shall be drawn with a thicker line. This is an aid to the quick assessment of height difference and the overall shape of the terrain surface. Where an index contour coincides with an area of much detail, it may be shown with a normal contour line. (italics added by me)

That's from here.

Matthew
Cincinnati
 

In addition, something I typically do is to not make a small knoll be an index contour, even if it happens to be at the index level, because it makes the knoll appear more prominent than it ought to. The type of "de-indexing" shown in the example is not common, but is, as Matthew points out, consistent with the standard.
 

I'm glad to learn that the mapper didn't break the rules.
 

It's legal, but I wish they wouldn't do it. Makes it harder to get a sense of the lay of the land. See the map from the WMOC final in Austria. I think it would have been better to keep the index contours in the detailed area.
 

The IOF Map Commission's Mapping Instructor's Kit (which someone needs to make it into a powerpoint or something---the overhead slides and instructor's notes are impossible to use together online)...

Anyway, the Instructor's Kit has a really mind-bending exercise for index contours. I guess this is something mappers learn but it still messes with my brain.

And I really agree with Peter on that map---not having the index contours makes it hard to read.

Matthew
Cincinnati
 

I think I have argued before that the "mind-bending exercise for index contours" in the instructors kit is completely wrong and not at all how index contours are intended to be used. Slope lines should be used instead. (Some mappers from nordic countries think that slope lines could only be used with depressions and form lines but that's also wrong.)
 

I think Erik Sundberg has it exactly right with index contours and slope tags. Map reading should be a "quick picture" not a logic puzzle. I also like jabbing at the big boys (and girls) when they deserve it. :-)

Like JJ, I am always on the alert for knolls exaggerated by the index contour. It is additionally aggravating when the knoll turns out to be flat or low, and more appropriately shown with a form line. ISOM 111- "smaller or flatter knolls shall be shown with form lines" Unlike full contour knolls, this form line passage does not reference the absolute contour level, which I agree with.

The WMOC finals map is the most extreme example I ever noticed of dropping the index lines, and certainly worthy of discussion. However, I am completely with the mapper on this one, in fact I think he/she did a great job.

For my eyes and brain, index contours would wreak havoc in this area, cluttering and distorting the the map image. In this context, I think the portrayal of the detailed terrain shapes takes priority over height and slope assessment. This is control finding terrain, not route choice terrain, and I think the index contours around the edges still do a reasonable job at showing the big picture.

This terrain is simply difficult(but great), and no style of drafting will make it easy. If the index dropping principle holds up here, and I think it does, it will hold up anywhere.

I think the mapper/draftsman also used an appropriate amount of slope tags, more than just on depressions, but not so many that they became a cluttering element.
 

I'm 100% with the Erics with regard to the exercise. Completely evil and misguided. The first two sentences show that the person who created this has a warped view of what index contours are all about.

As far as I'm concerned, there are two basic purposes that they serve:
1) Allowing you to more quickly gauge climb on a hilly route (because there are too many contours to count individually)
2) Providing a way to easily follow a slope with your eye to determine whether a traverse route should be going uphill or downhill.

I'm undecided on the Austrian map (because of purpose #2), and if I were drafting it, I'd look at it with and without index contours to see which I liked better (easy enough to turn them on and off using 0CAD). I'll note that when I originally drafted Pawtuckaway, I intentionally did not use index contours because I thought they would add little value, but provide a distraction (Tony Federer added them when he revised the map).

If it's not clear which way the slope goes, then add slope tags.