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/

[Andreas Plesch]

https://pds-imaging.jpl.nasa.gov/data/msl/M...CES_PDS_SIS.PDF

section 3.9.2 has a discussion on easting which I think provides a start of an explanation:

" These “easting” meters at a given latitude are related to true meters at the equator by the simple formula:

map_meters = true_meters / cos(φ)

where φ is the planetocentric latitude. "

[ hey, one can use html entities in the forum ♥ https://www.w3schools.com/html/html_symbols.asp ]

It looks like the provided meters are true meters (at the equator) and the projected easting has map meters since:

4590298.31635 = 4354494.086 / cos(18.4446271 degrees)

This is getting closer to 4590877.824, with still ca. 580m of a difference.

The gdal computed easting assumes a sphere with a radius of 3396190 m. Perhaps the reported true meter easting takes into account a local radius because these are also listed in the geojson, in the radius field.

The gdal computed easting and northing is consistent with the simple formulas given in section 4.4.2.4:

" planetocentric_latitude: Latitude of the point, measured using a planetocentric system.
Planetocentric coordinates are measured as angles from the center of the planet. Latitude (φ_pc) is
computed from northing (x) using the formula:
φ_pc = x / Re • 180 / PI
where Re is the ellipsoid radius, or 3396190 meters.

longitude: Longitude of the point. Longitude is computed from easting (y) using the formula:
θ = y / Re • 180 / PI
"

[ This is good and so simple that I can probably directly plot the traverse geojson on my equirectangular basemap ].

Since the northing matches, Re is 3396190 m. In addition to the correction to true meters from map meters easting, something else seems to be done to the easting. But what ?

[markril]

4590298.31635 = 4354494.086 / cos(18.4446271 degrees)

This is getting closer to 4590877.824, with still ca. 580m of a difference.

Solving for the latitude using the target easting we get:

18.46629998 = acos(4354494.086 / 4590877.824) degrees

Curiously, this number shows up as the center latitude of an orthographic projection in this metadata file. The relevant snippet is this:

CODE

<mapproj>
<mapprojn>Orthographic</mapprojn>
<equirect>
<stdparll>18.4663</stdparll>
<longcm>77.4298</longcm>
<feast>0.0</feast>
<fnorth>0.0</fnorth>
</equirect>
</mapproj>

The TIFF file this refers to is described on this page as the Derived JPL Surface Operations Mosaic:

Derived JPL Surface Operations Mosaic: This mosaic was finalized at JPL specifically for surface operations and mapping. It use the same individual images provide here but optimized for visual purposes using slightly differing seamlines and a 120 pixel blend across the input images. It uses an orthographic map projection centered on the landing site (77.4298 Lon, 18.4663 Lat). Download GeoTIFF (3.2 GB LZW compressed)

Thanks for bringing this up because it looks likes this may have solved a problem I've been working on, too.

Mark