Add draft recipes for Mercury, Mars, Venus, and Pluto

[PaulWessel]

All based off the recipe for the Moon since all are TIFFs. There may be some minor adjustments needed as we actually try to build the collections. Here are a few related questions:

• NASA likes to do things like "N pixels per degree". So of N = 64 then the grid spacing is 56.25 arc seconds. This does not fit very well with our 1, 3, 10, 15, 30, .... naming scheme. Of course, the very first downsampling can become one of those, in this case 01m and then up. But I dont like to name things like @mercury_relief_56.25s_g|p, for instance (and the gmt code decoding from strings will need to check for these decimalized numbers). One option would be to use a special flag for "the highest resolution", say, @mercury_relief_full_g|p so the users would not need to know what that resolution is. Of course, if you just say @mercury_relief in grdimage or similar then you get the best resolution needed to make the map at the default DPI (GMT_GRAPHICS_DPU) but if you want to do grdmath or request the full resolution then you need to specify that.
• GMT has ellipsoids (i.e., spheroids) for the planets, but they all say 2000 (official values at that time). I notice there are discrepancies between those values and the radii listed on the NASA site for the various data sets. We use those radii to convert filter widths in km to spherical angles for that planet, so that works well. But then, if you are to make maps of Pluto in GMT you need to do --PROJ_ELLIPSOID=pluto and then there is a tiny mismatch between the two radii. Not sure if this is important. But while we have > 50 ellipsoids for the Earth, we only have one each for the planets. Do we add pluto-NH (New Horizon), mercury-M (Messenger), Moon-L (LOLA) etc.?

I think it makes sense to add all these terrestrial planets at the same time and not just do the moon.

Comments, please.

[maxrjones]

@mercury_relief_full_g|p seems useful.

Not sure about the second question. How large are the differences and how much work would it be to add new radii?

[PaulWessel]

We need a planetary scientist to tell us what to do here - I am sure there is a simple answer and even official names for newer spheroids etc, I am just clueless about those. Workload is to add one ascii record to gmt_ellipsoid.h per new info, so trivial.

[joa-quim]

I've not yet looked at this discussion with detail, but shouldn't we base the spheroid/ellipsoid definitions in +proj=... strings?

[PaulWessel]

I've not yet looked at this discussion with detail, but shouldn't we base the spheroid/ellipsoid definitions in +proj=... strings?

Well, this is no different than for Earth, I think. We have WGS-84 as the default ellipsoid but there is only one default and there is not a default ellipsoid for each planetary body. In fact, we only have one ellipsoid per body, e.g., tail end of gmt_ellipsoids.h:

        {"Moon", 2000, 1737400.0, 0},
        {"Mercury", 2000, 2439700.0, 0},
        {"Venus", 2000, 6051800.0, 0},
        {"Mars", 2000, 3396190.0, 1.0/169.894447224},
        {"Jupiter", 2000, 71492000.0, 1.0/15.4144027598},
        {"Saturn", 2000, 60268000.0, 1.0/10.2079945799},
        {"Uranus", 2000, 25559000.0, 1.0/43.6160409556},
        {"Neptune", 2000, 24764000.0, 1.0/58.5437352246},
        {"Pluto", 2000, 1195000.0, 0}

We could possibly just revise these 20-yuear-old values to whatever NASA think is reasonable today. For instance, all those dataset specify the radius and I think the moon matches but the others are a bit different.

[Esteban82]

We need a planetary scientist to tell us what to do here - I am sure there is a simple answer and even official names for newer spheroids etc, I am just clueless about those.

I know one that I could ask if needed.

[PaulWessel]

Please do. I have asked Sandwell who has passed it along to a more planetary type but still waiting.

[Esteban82]

Do you know Belaya from the forum?

[PaulWessel]

Nope, no idea who that is.

[Esteban82]

I received an answer. She told me that they used the ellipsoids from 2000. And also she can ask some collegues from NASA this week.

[PaulWessel]

OK, so perhaps we just stick with those we have then for now, unless you get more recent info later. Thanks for checking.