[HOT] open geology map

Hazel hlhj2 at srcf.net
Thu Mar 12 21:03:03 UTC 2015

Dear All,

Can we again discuss putting geological data into OSM? Specifically, I'd 
like a recommended way to tag fault lines and surface geology polygons.

This e-mail assumes the reader knows nothing of geology, apologies to 
everyone else.

First, the usecase: geological data saves lives in natural disasters, it 
is useful for common activities like agriculture, and it is interesting 
in its own right. It can also be usefully collected by amateurs.

I am not suggesting that OSM should produce disaster risk maps, or 
recommendations for farmers. I am saying OSM could collect the data that 
would allow experts to quickly and easily make these things.

Using OSM contours, they can work out areas of flood risk and tsunami 
escape routes. Using contours and and basic geological information, they 
can work out areas of landslide risk (landslides kill more people than 
volcanoes or floods or earthquakes, but they kill a few dozen at a 
time). If we map faults, they'll know more about where earthquakes are 
likely to happen (you know the photos of roads after earthquakes, offset 
by a few centimeters? The fault is the plane where the offset happens, 
and earthquakes use the same faults over and over again). If you map 
areas of shallow bedrock vs. unconsolidated sediment, you know which 
areas may suffer soil liquifaction in an earthquake.

https://en.wikipedia.org/wiki/Soil_liquefaction soil liquifaction

Technical infodump:

To make a geological map, you map areas with similar surface rock or 
sediment2. You describe them (anything from field IDs like "greenish 
rock #2" to detailed technical descriptions) and give them proper names 
(e.g. "the Tunbridge Wells Sand Formation").

Having mapped the boundaries between different rock types, you can also 
trace faults and the line of folds in the rocks. These all obviously 
exist in 3-D, but are usually represented on 2-D maps. Just mapping the 
2-D trace is enough for many purposes.

If you want to add more information about the third dimension to a two-D 
map, there are conventions for that. You specify a line (along the axis 
of the fold, or on the steepest line down the fault plane or boundary 
plane). You map the direction of this line. Then you measure the angle 
between the line and the horizontal, and write in on the map (next to 
standard symbols: for a plane, a T-shape, and for a fold axis, an X with 
two or three of the lines turned into arrows pointing in the two or 
three downhill directions).



Planes on either side of a fold:

This is actually fairly easy to explain in 3-D, but not in 2-D, and I 
don't know of a good video. We could make one.

Let's look at the Weald area of the UK, since it is well-mapped.


"Lower Cretaceous" and "Upper Jurassic" describe age (lower means older)
"rocks", "chalk" and "sandstone" describe rock type
"sands" and "clays" describe sediment type
"Purbeck Beds", "Ashdown Sand Formation" and so on are proper names of 
groups of rocks/sediments. These names are hierachical, like taxons, and 
are in databases (for the Chalk Group that forms the White Cliffs of 
Dover: http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=CK).

The cross-section may help make the 2-d map make sense.

To see how faults and folds (synclines/synforms, that sag, and 
anticlines/antiforms, that hog) are mapped as lines, see this map:
(just gives rock ages, not type).

Faults are usually much more obvious on small-scale maps than they are 
on this map.

For sediments, there exist multiple soil classifications, with mappings 
between them, and OSM could support them all, but the classes we have 
(sand, gravel...) would be enough to start with.

QGIS is increasingly used for geological mapping, so it works 
increasingly well with many other geological tools. QGIS is already 
well-integrated with OSM. The barrier for geologists new to OSM to 
upload their maps is therefore low. Classes of students could do it.

End infodump, requests for clarification and corrections welcome.

Could anyone suggest a set of minimal changes that would make it 
possible to enter data like this? As I said, just having a recommended 
way to enter a surface geology polygon, a geological contact line 
(between two polygons), and a fault line (with optional dip direction 
and inclination) would be very useful.

Pseudo-3-D perfection would also allow keeners to input the contact 
between two rock formations (line, with dip direction and inclination) 
and input folds (line, with dip inclination), but this can also wait.


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