What do the tick marks on closed contours indicate?

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Discussion Overview

The discussion revolves around the interpretation of tick marks on closed contours in topographic maps, specifically whether these contours indicate hills or depressions. Participants explore the implications of contour markings in various contexts, including physical mapping and geological features.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants suggest that the isolated curves may indicate hills based on the surrounding landscape, but others argue that this is not certain and that they could also represent dips.
  • One participant references a source indicating that depressions should have tick marks on the low side, while mountains do not, suggesting a standard practice in cartography.
  • There is a question about the specific type of tick marks used and whether they are applicable to depth charts, with some participants expressing uncertainty about their usage in different contexts.
  • A participant describes a specific landscape and hypothesizes that the closed contours are likely raised limestone spires rather than depressions, raising questions about local geological conditions.
  • Another participant notes that closed contours representing depressions are distinguished by tick marks on the downhill side, particularly in karst landscapes.
  • There is a discussion about the treatment of depressions in underwater topography, with references to how tick marks may or may not be used in such contexts.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of tick marks and closed contours, with no consensus reached on their definitive meanings or applications across different types of maps.

Contextual Notes

Some statements rely on specific cartographic conventions that may vary by region or type of map, and there is uncertainty regarding the presence and significance of tick marks in digital mapping tools.

DaveC426913
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I'm trying to make a physical 3D map of this landscape.

These isolated curves (red) have no altitude on them, so it's up to me to interpret.

How do I know if these are hills or dips?

Can I gather from the rising landscape (marked in magenta) that these are probably hills?
Can I be sure?
test.png
 
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Huh. I don't see how you COULD know. It certainly seems likely that they are above the surrounding territory, but that's not a given. It would be perfectly correct to draw the landscape that way if they were dips.
 
Google Earth or Map? Usually you can just "feel" it from the contours. Interesting.
 
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hutchphd said:
According to this source
http://geokov.com/Education/mountain-pass-cliff.aspxIf it is a depression it should have tick marks on the low side. I do remember seeing these on charts. The mountains have no tick marks.
Those cartographers are careful folks.
Interesting. What kind of tick marks? Can you show an example?
 
phinds said:
Interesting. What kind of tick marks? Can you show an example?
It's at the bottom of the page on his link.
 
But I don't think depth charts use the tick marks for bottom contours. It would probably make them too "busy" but I don't know that they differentiate them. They are color coded for shallows so its usually easy to tell.
Anybody know?
 
hutchphd said:
According to this source
http://geokov.com/Education/mountain-pass-cliff.aspxIf it is a depression it should have tick marks on the low side. I do remember seeing these on charts. The mountains have no tick marks.
Those cartographers are careful folks.
Thank the heck out of you! Of course!

(Alas, these are digital maps from Toporama. I'm not sure if they're actually showing that symbol. And it's very difficult to verify.)Now that I think about it, depressions are a comparatively rare artifact - especially if not water-filled or at least marshy. 99.9% of small, closed-contour land features are going to be bumps.
 
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  • #10
In the top left is a gully, cut into a scarp. It feeds an alluvial fan at the foot of the scarp, that fan has a gentle slope and covers the local bedrock. In the lower right, the distinctive lack of chevron patterns made by streams in valleys, makes it look like a karst landscape. I would expect sinkholes with such a geology, but those isolated and closed contours appear to be on steadily sloping ground, which suggests they are more likely to be raised limestone spires = hills.
I wonder, what is the local rainfall?
 
  • #11
From this map, it appears to be treated the same way.
hutchphd said:
But I don't think depth charts use the tick marks for bottom contours. It would probably make them too "busy" but I don't know that they differentiate them. They are color coded for shallows so its usually easy to tell.
Anybody know?
I checked a chart online and it appears that depressions in the local topography are treated the same underwater. It's been a while since I did any cartography but think about it this way. If there was no water, where would you start creating tick marks? Certainly not at the 'shoreline' that doesn't exist.

https://www.mindat.org/glossary/depression_contour
A closed contour, inside of which the ground or geologic structure is at a lower elevation than that outside, and distinguished on a map from other contour lines by hachures marked on the downslope or downdip side.
 
  • #12
A bit of clarifying colour banding helps clear it up.

test2.png


I'm glad I took the time to colour the whole thing. I would have been hopelessly lost pretty quickly if I hadn't.
 

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  • #13
Closed contours that represent depressions have tick marks on the downhill side. This is only true of closed contours. You can see such features on karst features such as sinkholes. Being a caver, I often peruse topo maps online for such features to find caves. You can find many such features on topo maps of N. Florida just south of Tallahassee where there are many sinks.
 
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DBO said:
Closed contours that represent depressions have tick marks on the downhill side. This is only true of closed contours.
Under what conditions are contours not closed? 😕
 
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