Most of this blog deals with mapping and rapid transit issues, particularly street railways and trolleybuses. I would like to step away for a moment from the rapid transit part and dealing with another mapping issue, mapping other worlds. Mars has always been interesting to me on account of the supposed canals. Perhaps Martians also had some form of rapid transit along the canals? At any rate, thanks to the internet and interesting map programs as GRASS 6.4.1 which does all sorts of hydrology and other operations, let us see what Mars has to offer?
The following source shots were probably unzipped from a NASA website by myself:
The map below was produced from a shape file of the Mar's surface. I produced a map in ARCGIS and then transferred the program to GRASS. The two points of observation are pointed out. One is in the center of a crater where one of the Rover modules landed in 2004. Its' approximate location is -260521.36, -87339.29. I did not get the exact location*. Notice the "river like" crack south of the crater that goes to the east. I also picked as a reference point -251316.35 and -1044693.55 which is sort of a projection of some kind near the south side of the crater..
As a experiment: What would a person at this two locations see on Mars? Here we are asking about visibility. Second, how would water flow in the "river" to the south of the crater? To answer this, we will use GRASS 6.4.1.
* The location of the rover landing is handled by a separate shape file and I will need to investigate this further. The coordinate system by default in ARCGIS and GRASS is not the same. I picked the rover location at he near center of the crater randomly since the surface appears to be relatively flat. The picture below comes from NASA. It shows sunset over the Gusev Crater which is the crater that is shown below that has a "river " extending from it's southern section. The river is called "Ma-adim Vallis".
In the map above, I used the GRASS contour generator. My range is from about +4000 m to -4000 m in 1000 m sections. Notice the river is stands out clearly from the surroundings. The colors are coded according to elevation. Essentially, this map is a elevation file that is coded according to altitude. This was done automatically by GRASS. Do you see the red circle disk with two projections on the east side near the "river" Ma-adim Vallis.. This high area will also be a frame of reference. What would an observer see at both locations if the observer is 1.5 meters high? This is the raster - terrain analysis - visibility sequence.
I cannot explain why I got this triangular shape. When I do the analysis at above 200,000 meters, the system crashes completely. Perhaps this odd shape is due to a computer problem and not a visual result. The dot at the north point of the triangle is the frame of reference.
In the map above, the yellow-green triangle is the visibility from the center of the projection that has the yellow dot in the red disk. I do not know why I get such a pattern.
In the map below, the GRASS 6.4.1 hydrology program was used to indict flow areas downhill from a rainstorm. The program will ask you to supply two parameters or more dealing with the length of the vector produced and the number of "cells" that it covers. Here you will need to try your luck in case you are like me and do not know what you are doing.
The above map was produced in GRASS 6.4.1 using the raster-hydration program dealing with water flow and is the same as the picture above except for different settings. You can see the shape of the Gusev crater and the area where the river is. The water flow is perpendicular to the river and not along it. The walls of the crater produce water flow also perpendicular to the walls.
And I almost forgot, here is the visibility at a random location near where the rover landed in 2004.
Notice at the right side of at the bottom of the crater there appears to be a projection of a low area which appears to be the beginning of the "river".
This map was produced in GRASS using the slope aspect. Various derivatives can be derived from the slope of the elevations. I believe this is a dxdy partial derivative.
Below is a map that used GRASS to produce contour lines at 500 m intervals from -4000 to +3500.
Notice how much more information is provided. The distance between two contour lines is 500 m and you can see how steep the slope is to the "river".