In a previous set of posts, I showed a summary map of the mid-latitudes of each hemisphere of Mars, showing the Mars Express HRSC tiles and the Souness glacier-like forms.
The QGIS project file for the southern hemisphere using only tiles with DTMs at resolutions 50m and 75m currently takes over 15 minutes to load, on a quad-core i5 PC with 32GB RAM.
To make it more managable I cut it down to the region of Mars east of Hellas Planitia region of Mars. This allows me to add it a few more fields with lower resolution HRSC DTMs.
The overview looks like this:
|Hellas Planitia is on the left (the terrain is intrinsically bright). Units are in metres in the equicylindrical projection at standard latitude of 40° (approx 2371000m). 1 degree of latitude is approx 60km. 1 degree of longitude is approx 45km at 40° latitude.|
|HRSC DTM elevation colour ramp for the majority of fields (except a few within Hellas which start at -8000m)|
|A crater at ~ 97°E, 37.5°S, in the HRSC tile h6437. A large number of Souness GLFs are seen in a relatively small area around an eroded crater in Hellas Montes. There are several HiRISE anaglyphs: ESP_020860_1420, PSP_006672_1420, PSP_005670_1415.|
|Nadir image alone.|
|The same region, showing the areas of high values of the classifier fuction for extent areas (blue) and head areas (red), overlaid of the nadir image for tile h6437.|
|Screenshot from Google Earth showing CTX imagery of the same area. |
Elevation and hillshade
One of the problems with the visualisation as it is, is that the green channel is overloaded, because of the way the greyscale nadir image is blended with overlaid topography encoded with a colour ramp.
Below I show a pure colour ramp for the elevation only, as well as the colour ramp overlaid with a hillshade (blended with 'Hard Light' in QGIS):
|The colour ramp used for elevation (m relative to datum)|
|Elevation only (MOLA/Mars Global Surveyor)|
|Elevation with a hillshade layer blended.|
Near Reull Vallis
|Classifier function overplotted|
|HRSC nadir image|
|With the classifier.|
|The southern portion of Greg, showing gullies in the southern rim.|
|With the classifier.|
Working with my maps of a segmented DEM showing average slope direction and magnitude, and curvature with arrows, I noticed it broke once I upgraded to QGIS 2.12.
Unless there has been some change in the syntax of expressions that I didn't get from the QGIS 2.12 changelog
I suspect a bug.
I made a minimal example, which I shared via GIS Stack Exchange
and QGIS issue tracker
|QGIS 2.10.1. The width of the lines are proportional to the magnitude of the slope, and the colours are proportional to the 'longitudinal curvature' with blue for concave slopes and red for convex. No prizes for guessing the location since it's labelled on the maps at the links above.|
|QGIS 2.12. Styling is lost for the slope magnitude and longitudinal curvature, but slope direction is still shown. |
I saw the National Library of Wales had digitised a map of Wales produced circa 1574 by Humphrey Llwyd, and uploaded a copy to Wikimedia commons
I decided to try georeferencing it to a modern map based on OpenStreetMap data using the QGIS georeferencer:
|Overlaying 1574 map with transparency shows the variability in the accuracy of the map. full-size version|
I chose a variety of places around Wales and the border areas shown on the map, including Cardiff, Newport, St. Davids, Aberystwyth, Holyhead, Liverpool, Shrewsbury, Hereford.
The result looks more accurate than I expected, with Aberystwyth and Shrewsbury being almost exactly referenced, and several other places with small errors.
According to the author Graham Robb
, Whitchurch in Shropshire was the prime longitude meridian for the British Druids in the Iron Age, and is named as "Mediolanum" by Ptolemy in the 2nd century. Aberystwyth (or more exactly, the Pendinas hillfort) was also an important location within their navigation system apparently, being both on a longitude meridian and latitude paralell.
Update: QGIS layer blending
Changing the layer blending method to a "Hard light" overlay rather than semi-transparency makes for a slightly clearer result I think:
As an example, in part of the HRSC tile h2224, Colin Souness in his PhD work found a number of glacial like forms in the northern wall of the crater.
Greg crater is located at 38.5°S, 113.1°E and has a diameter of 66km.
The southern wall of the crater tends to be covered in gullies. There is a detailed summary
of it on the website of the Planetary Science Institute, Tucson, based on the W. Hartmann et al paper
Now looking at the classifier, labelling extent areas above ln(K) > 10 and head areas above ln(K) > 12. I have used "Addition" blending in QGIS to show areas that are classified in both in purple.
There are potential glacier source areas above the Souness objects, but there are many other areas that are also marked out as source areas that do not have glacier like forms catalogued by Souness, such as lower down the north face, the east face and the south face of the central mountain.
Google Earth can be useful to visualise Mars, selecting the CTX image mosaic (the 'context' camera with 6m resolution from Mars Reconnaisance Orbiter):
It is also possible to see where HiRISE coverage exists (red outlines):
Here is the southern mid-latitudes of Mars, in the same colour scheme as the previous post
showing the northern mid-latitudes:
|The lowest elevation area of Mars, the Hellas basin has areas of bright terrain which makes the colour scheme a little confusing. Download larger version.|