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 Post subject: Re: Creating a continent
PostPosted: Mon Apr 24, 2017 4:09 pm 
Sanci
Sanci

Joined: Wed Jul 24, 2013 4:40 am
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That's very clever, I'll try it myself.


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 Post subject: Re: Creating a continent
PostPosted: Thu Apr 27, 2017 7:45 pm 
Smeric
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Quote:
A neat thing about the donjon maps is that there are many depressions in the terrain that are above sea level which would of course form endorheic basins, often filled with lakes and seas, and of course providing river sources where the basin is filled to its rim, not to mention the exciting possibilities of having catastrophic release floods, ala post-glacial northern hemisphere everywhere (Bonneville, Missoula, Doggerland/English Channel, the whole evolution of the Great Lakes, many similar glacial lakes in Siberia).

I just wonder how difficult it would be to create a small program that, given a height map, would simulate rainfall and use it to trace rivers and place mid-continental lakes.

(Not very difficult, I presume, but I'm too busy procrastinating now to give it a try.)

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 Post subject: Re: Creating a continent
PostPosted: Thu Apr 27, 2017 9:04 pm 
Sanci
Sanci

Joined: Fri Nov 14, 2014 8:02 pm
Posts: 35
Location: Delaware
Pole, the wrote:
Quote:
A neat thing about the donjon maps is that there are many depressions in the terrain that are above sea level which would of course form endorheic basins, often filled with lakes and seas, and of course providing river sources where the basin is filled to its rim, not to mention the exciting possibilities of having catastrophic release floods, ala post-glacial northern hemisphere everywhere (Bonneville, Missoula, Doggerland/English Channel, the whole evolution of the Great Lakes, many similar glacial lakes in Siberia).

I just wonder how difficult it would be to create a small program that, given a height map, would simulate rainfall and use it to trace rivers and place mid-continental lakes.

(Not very difficult, I presume, but I'm too busy procrastinating now to give it a try.)


That's called Wilbur. It's free and the cartographers guild is full of tutorials on it.


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 Post subject: Re: Creating a continent
PostPosted: Mon Jun 19, 2017 3:38 pm 
Avisaru
Avisaru

Joined: Wed Dec 05, 2007 10:27 pm
Posts: 250
Wilbur's a bit less capable than that, but still nearly indispensable, even if you have professional-grade GIS software. Basically, the closest that Wilbur comes to that precipitation and water movement model is a D8 flow accumulation model. It basically just counts the number of pixels contributing flow to every pixel on the screen based on an algorithm where all the downflow from any pixel is passed down to the lowest elevation of its 8 neighbors. This isn't perfect by any means and it fails to take into account deserts and groundflow infiltration/exfiltration, but Wilbur's Incise Flow model uses it to good effect to create eroded landscapes, and the River Flow texture uses it to… passable effect to place rivers. Just edit out most rivers originating in desert regions.

Quote:
I think that the other continents should be finished before you actually find the wind, water and climate patterns, because continent shape and size are really important (think of the Gulf Stream Current and how it neatly fits with the shape of North America.) Island chains between and far away from large landmasses will also affect the patterns.


This point is largely correct, but…
The rough outline of Europe surely has an effect on the North American climate. At least on the east coast. On the other hand, even if you modeled the climate with a good GCM, I doubt if the location and height of the Alps would have a perceptible effect on the climate of North America. At least not beyond the noise of sensitivity to initial conditions and step-size effects on the model.
The effect on the climate in North America of leaving Australia out altogether would be negligible, although it might have knock-on effects on the length and depth of El Niño cycles and western North America's famous long drought 'cycles'. These effects would be difficult to approximate without a GCM. Roughly equivalent to trying to accurately model the Milankovitch Cycles of a planet in a different solar system. If you're just trying to create a plausible climate map using guidelines like Geoff Eddy's Climate Cookbook then you just need nearby continents. For North America, what you would need is South America and Europe. You don't even need to know those in any great detail.

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 Post subject: Re: Creating a continent
PostPosted: Thu Jun 22, 2017 5:26 pm 
Avisaru
Avisaru

Joined: Wed Dec 05, 2007 10:27 pm
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Here is a little something I cooked up, though it's not a perfect fit to your guidelines. I'll describe my methodology, as it could prove useful to others.

First, I went to the USGS EarthExplorer site. I used to use seamless.usgs.gov, but that seems to have gone away :x . To download data, you need to be logged in, but setting up a profile is free. So, why not?

Under the Search Criteria tab, I selected the Address/Place tab and entered "stewart island" and tapped Show. Click on "Stewart Island / Rakiura, Southland 9818, New Zealand". Zoom in a lot and tap Clear Coordinates. Click on the map to define the outline of the area you want to use. Now select the data sets tab. In the hierarchical list click on Digital Elevation, then SRTM. Under SRTM check SRTM Void Filled and SRTM Water Body Data. Then click on the Results tab. You should get four elevation data sets, and four water body data sets. Click on the footprints for each of these to insure that they cover the desired area., if they're where they should be click on the download button for each data set in the SRTM water body data. Then click on each of the SRTM void filled elevation data sets, in the resulting window, click the Download button next to TIFF, although it looks like Wilbur can handle DTED.

Now that you have the data, this next step will require QGIS. You can get QGIS free here.
Next add the four raster layers to QGIS.From the main menu, select Raster>Miscellaneous>Merge…
Select the four input files, then browse a location to place the resulting merged elevation file. I named it merged_elevations.bt, and selected VTP .bt as the type. Now, I like to double click on the new layer in the legend area to open properties. In the Style tab, under load min/max values, I select Min/Max and Actual(slower), then hit Load and OK. This just makes the display look nicer.

Next, I'm going to clip the coverage area of the final raster to just the island. Start by clicking the New Shapefile Layer tool. This will create an empty new vector graphics object. Give it a name and set its type to polygon. We will fill that with the boundaries of our desired area. With your new shapefile selected, tap Toggle Editing in the Digitizing toolbar. This will allow you to define the area of the island's raster. Click the Add Feature tool in the Digitizing toolbar. Now click in the water areas around the island to create a new polygon encircling the island. As a guide, you can create a "landarea" raster using Raster Calculator under the raster menu. Simply set the expression to "merged_elevations > 0.0". Place that below the shapefile layer as a guide. Tap Toggle Editing again to finish your polygon.

Now pull down Raster>Extraction>Clipper… to actually clip the elevations. Set the input file to your merged_elevations, set the Clipping Mode to Mask Layer with the shapefile you just created as the Mask Layer. Name the resulting file to something like "clipped_elevations".

Open Wilbur and open up clipped_elevations.bt.
Use the paintbrush with value set to something tiny like 0.001 and the Operation set to Maximum to paint over areas below zero within the island. These may be a fault in Wilbur, I'm not sure. I don't see them in QGIS.

Next Surface>Locate>Flip Vertically to get roughly the same arrangement as has been used previously in this thread.

We'll need to use an exponent operator to contract the mountain areas. Realistically, most continents would have a bit less highland, so that's what I'm going to do here. Select>From Terrain>Height Range… from Minimum: -1 to Maximum: 0. With that selection, Filter>Fill>Set Value… Set Value: -1. Otherwise the exponential operation tends to mess things up. Deselect. Next, Filter>Mathematical>Exponent… Set the exponent to 2 for the Land(above sea level), and 0.5 for the Sea(below sea level), 0 for the Sea Level. Preserve Height should be set to Absolute Low set to -1, Absolute High set to 791.

Use rectangle select to select an area very close all around the island. Now, Surface>Crop to Selection.

Finally, to place this where it belongs, sort of, Surface>Find Min/Max… Top: 60, Left: 80, Right: 140, Bottom: 0. File>Save As… set the type to Binary Terrain Surface(*.bt). Name it something like "squared_elevations".

Back to QGIS. While we used Wilbur to assign the new coordinates which were stored in the VTP .BT format, but the projection is hardwired as UTM. So, when the elevations file is loaded we need to reassign a WGS 84 latitude-longitude "projection"(Raster>Projections>Assign Projection… elevations.bt as the Input file, EPSG:4326 as the Desired SRS. This will be an equirectangular "latlon" coordinate system). Looking back on it, I think I can manage the projection described in this thread using georeferencing and probably a polar equidistant projection.

Finally, I'm going to use the Raster Calculator one more time. I'll use the expression: 4*squared_elevation + 2.375*unsquared_elevations to give me a range. That will give us our final elevations.

For now, I'm going to leave it at that. There's a lot more you can do, particularly when you've created a vector representation of the landmasses.

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 Post subject: Re: Creating a continent
PostPosted: Mon Jul 17, 2017 6:57 pm 
Avisaru
Avisaru

Joined: Wed Dec 05, 2007 10:27 pm
Posts: 250
A picture to go with all of that.
Image

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 Post subject: Re: Creating a continent
PostPosted: Mon Jul 17, 2017 7:53 pm 
Lebom
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oh, my! Now I'm jealous! Very, very nicely done!!

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