Mapper's Challenge #30 — July 2020 — Contours and topography

Posted by zhenkang on 15 July 2020 in English (English).

Hello there. Hope the OGF community is doing well.

I appreciate the responses so far for the last challenge. I hope more people will jump in, especially for this challenge, which is particularly interesting.

I have successfully added a few bus routes in Bakdep, but beyond developing the core area, I have not been able to develop other bus routes beyond the core. However, once more areas have been developed, I will create more.

Mapper’s Challenge #30 - July 2020 - Contours and topography

Initially, for this month, I wanted to do Urban Periphery: The transition zone between inner-city areas and the surrounding countryside, as the one who suggested the challenge (Thilo) commented that it is something that “I’ve seen rarely done well in OGF.” However, I feel that will be putting the cart before the horse, so I decided to switch to this challenge on contours.

Contour mapping is not that complicated; you just need at least some basic JOSM skills. A good guide explaining contour mapping can be found here. You can also look at the cycle map in OSM for inspiration.

I encourage you all to do some basic contour mapping around the city or town you are developing at the moment. If you are advanced enough, you can try a mountain range through your country.

For this challenge, I am going to map a few hills around Bakdep and Phong Thinh. Currently, these hills are mere patches of forests, but these will be developed soon enough.

It may take some time to see the results (NOTE: DO NOT upload your contours directly to the database!), so I encourage you all to be patient. If you are more organised, you can do contours in degree squares, like what some experts on this did.

Comment from Toadwart on 15 July 2020 at 13:25

I was at first tempted to ignore this challenge. The topo layer is really inconvenient compared to the map layer. Especially as my first attempt years ago was spoilt by the fact that it doesn’t really show within built up areas, as black buildings overshadow the nuances of the topoMap. But I tried anyway.

There are some questions though:

  • When should I use the natural=coastline tag? Is it equivalent to ele=0?

  • Is ele in meters?

  • Do lines need to be a closed way?

  • Do lines need to be continuous? I made a 1100m line and a 1500m line, both quite long. At some part inbetween I added some more detail with a 1300m line. Does the 1300m line need to follow the other lines all the way or only where I deviate from the interpolated elevation.

  • What is the height within a closed way containing nothing else? Is it flat like a lake?

Comment from zhenkang on 15 July 2020 at 13:29

I set this challenge to prioritise topology before anything else: future challenges will also encourage topology.

For some of the qns:

  • At coastlines, yes ele=0
  • Elevation values are in metres.
  • Lines can be a closed loop or stop at the edge of where you are mapping, though it will make things pretty ugly there.

I think Luciano may give more satisfactory answers.

Comment from Luciano on 15 July 2020 at 14:01

When should I use the natural=coastline tag? Is it equivalent to ele=0?

I actually copy-paste my coastlines and lake-shorelines from an OGF layer to my contour file layer in JOSM. For coastlines, I explicitly add ele=0, for lakeshores, I add ele=. You can see this with my lakeshores in Makaska, for example, here:

Is ele in meters?

Yes. It is possible to use e.g. imperial system (US), but that would require changing settings on the server side. Best to keep everything consistent.

Do lines need to be a closed way?

Not at all. In fact break up lines is basically required when mapping smaller areas, unless you’re doing islands.

Do lines need to be continuous?

Not at all. In fact, HORIZONTAL interval is more important for the conversion than VERTICAL interval. So I make a practice of adding intermediate lines where slopes are shallow, e.g. flat areas. I’ll add short stretches of 5 meter (vertical) intervals where the distance between my 10 meter lines is large, or if an area is mountainous, I’ll neglect every other line, thus 20 meter intervals or even 40 meter intervals.

What is the height within a closed way containing nothing else? Is it flat like a lake?

This is a bit problematic. I have taken to adding diagonal “filler” contours on lake surfaces, to prevent “glitches” in the render (holes and canyons). Here is a screenshot from Makaska (north end of Lake Ohunkagan, directly west of the city of Ohunkagan):

Login to leave a comment