Skrifennow

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Cartographic decisions for dissertation

Sep 18, 2014

So now I've (almost) finished writing my dissertation, I'm putting in the various figures. I'm mainly using QGIS for this, most of the time the map composition feels more intuitive than ArcMap.

My previous post showed a three colour composite of showing image (or an aspect layer), elevation and slope, but I think that can be difficult to interpret, becasue a steep slope would either be blue, cyan, or magenta or white depending on what else is in the other layers.

So I'll be doing pseudocolour like this, using an image layer underlaid using transparency. I'm using meters in an Equicylindrical projection with 40 degrees latitude parallel. I think its nice to have metres as units for figures of individual objects/tiles, but maybe lat/long would work better for larger summary plots?

Elevation - overview east of Hellas

This is one of the areas I'm focusing analysis of the results for the dissertation. There are two tiles with 50m HRSC topography, and other tiles available at 75m, including the well-studied Crater Greg.

 Elevation

 Slope

I'm using a single colour ramp for slope at the moment, I think this is probably clearer than a spectral pattern.

Curvature layers

I wasn't sure which colour scheme to do this in, given they can be both positive and negative, but purple/green seems to be alright, as long as its clear which is positive.


An overview of glaciers in the mid-latitudes of Mars

Sep 2, 2014

The paper by Colin Souness et al. in 2012 (link) gave an inventory of Martian "Glacier-like forms"

Here is where they are found plotted on a layerstack of Mars Express HRSC DTM tiles, where red=nadir image, green = elevation, and blue = slope, falling back to MOLA topography in areas I don't have that tile because it didn't underlay any Souness GLFs. For MOLA I use aspect as a substitute for the image layer.

The Souness GLFs are numbered, but not all are plotted because QGIS automatically avoids plotting too many labels on top of each other.

The coordinates are Mars Equicylindrical, with a standard parallell of 40 degrees.

Northern Hemisphere (25 to 65N)




Southern Hemisphere (25 to 65 S)

 











Analysis of Crater Greg using LandSerf

Aug 28, 2014

I had some trouble with geomorphons, because I couldn't get the GRASS 7 extension to compile in Linux for unclear reasons, so I was limited to using the Windows version inside a virtual machine, and it decided to crash when presented with Mars data, although it was OK with Earth DTMs...

I have however been using LandSerf to generate curvature layers, and a feature classification, so here we are, for the well known (at least to Martian glaciologists) crater "Greg":

I hope you're not too confused by the use of transparency...

Units are in metres.

Some of the glaciers form a channel as they are constrained by topography, but not all channels are glaciers.....

Using longitudinal curvature calculated in LandSerf, showing negative curvature where a slope starts off steep and shallows as it goes down, which the Martian glaciers in Greg and elsewhere generally do, (Bryn Hubbard et al.):



Using RSGISLib to segment a layerstack consisting of nadir image, elevation, slope, aspect, and curvature layers:


Selecting all slopes above 15 degrees



Geomorphons of mid-Wales

Aug 24, 2014

Following the machine-vision based method of topographic analysis, "Geomorphons" by Stepinski and Jasiewicz, here's an example of the method applied to mid-Wales:

The authors argue that the method can make a useful analysis of topography at differing scales, in a better way than typical differential-geometry based methods do.

A close-up of the Aberystwyth area:

The method is available either as an extension to version 7 of the open-source GRASS GIS software, or, for small datasets, as an online interface.


After a number of hours, a wider search area can be made to match the scale of Stepinski and Jasiewicz's map of Poland in their paper , in which the authors used a DEM at 30m resolution at a search radius of 50 cells.



Hopefully I'll have something useful from this method applied to Mars in time to go in my dissertation....

Website: Space Informatics Lab - University of Cincinnati

Segmenting with LandSerf: Wales DEM

Aug 7, 2014

Using some more SRTM tiles, I will try segmenting the DEM of Wales, using the elevation, slope, aspect, and several curvature layers generated in LandSerf:


 Here is an example segmentation showing the mid-Wales area with RSGISlib with a minimum object size of 80 pixels (0.2sqkm):



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