Martian Cartography Options

[karolp]

I have recently freaked out a little bit about Martian maps of all sorts. And finally I was astonished with those highly detailed beauties that I list below. Nonetheless. some of them have huge inconsistencies (crater names) easily noticed when we compare the surroundings of Gusev crater. Enjoy:

http://www.ralphaeschliman.com/
http://planetologia.elte.hu/1cikkeke.phtml...arsmapinte.html
http://pubs.usgs.gov/imap/i2782/

[kymani76]

There is really no mystery here, only an age old geographical problem. Compromises need to be made in order to render spherical planetary surface onto a 2d surface.

First we need to see how HiRise images are produced. Camera images the surface of Mars while in orbit around the planet looking straight down, so images are really in orthographic (OR) projection. But as images cover very small part of spherical surface of the planet, it is possible to approximate it as a 2d picture in equirectangular (EQ) projection with central longitude and latitude near the center of the imaged area (in essence we treat this area as it would be flat). That's why different HiRise images come all with the same EQ projection, but varying central longitudes and latitudes. EQ projection also has true scale latitude defined, which means only at this latitude scale will be in true meters. On this map straight lines looking to be of the same length but located at different latitudes would have different lengths in reality. Also lines are that are always straight in EQ projection will be curved in reality (except at central long/lat point).

To get straight lines straight in reality as well as on the map you have to choose orthographic projection with central long/lat at the center of the area of interest. That's way Perseverance's HiRise mosaic used for driving comes in
OR projection with central long/lat at 77.4298/18.4663. This particular coordinate is at the center of HiRise mosaic as well as very near to a predicted landing spot. So what you get is a map with approximates very well the spherical surface as
a 2d Cartesian space. In essence you get x/y grid defined with parallel longitudes and latitudes. So when planning rover's traverse you can simply plot a straight line from point A to B and be sure the this line is also straight on the surface of Mars.
Pretty important when navigating a 2.2 billion dollar rover on Mars.

Coming back to geoJSON data and why it uses ER projection coordinates. Well OR projection really doesn't have a true scale, so if you want to measure a distance in meters you have to reproject it into ER with appropriate true scale latitude. Only then will your measurment be in true scale meters. Thats's why easting and northing in geoJSON file are given in ER projection with central long/lat at 77.4298/18.4663.

I hope this helps.


Mars in OR projection with central long/lat at 77.4298/18.4663. Note that extent of Perseverance's mosaic (yellow rectangle) is totally rectangular in this projection.