Deciphering the Documentation and writing a tutorial (edited)

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Hallo GMT group.

I am gradually getting the hang of the GMT syntax. I am grateful for your patience with me. The forum text editor keeps breaking my formatting, sorry. M.J.



(I have edited this post several times before clicking the commit button as I continue to cross reference the documentation. Please bear with some of my ignorance.)



Here, I read a 3600px by 1800px Plate-Carree png and reproject it to Orthographic over Cardiff.



gmt grdimage -JG+03/51/5 -Rd basic.png -png cardiff-JG-5



The image looks as I expect. Display SmileyFace !! Yay !



Scale set to 1 gives a 118 px square image ? What scale does that ?

Scale set to 5 gives a 591 px square image ?

I cannot work out the units GMT is working with here.

Is it pixels per centimeter? Is scale looking for centimetres ?

(I'm just coming right out and saying this is weird)

(unless GMT is focused on print media, in which case it's very logical)



The JG perspective options are a bit troublesome. I get nonsence results or errors.

As I add one by one the options described in the documentation, I try to work out the expected range of inputs without success.



This projection example is from the university of Hawaii GMT page:

-Jg|G<lon0>/<lat0>/<scl>|<width>[+a<azimuth>][+t<tilt>][+v<vwidth>/<vheight>][+w<twist>][+z<altitude>[r|R]|g]





I break it down here:



-Jg|G (upper/lower case indicates the type of scale to print?)



<lon0>=degrees (0 to 360 | -180 to 0 to +180)



/<lat0>=degrees (-90 to +90)



/<scl>|<width> unknown format | unknown units

[+a<azimuth>] Degrees (0 to 360 or other ?)

[+t<tilt>] (0 to 90 or 0 to 180 yaw? roll? pitch?)

[+v<vwidth> (unknown units unknown function) / <vheight>]

(is this oblique "/" a typo? is it meant to be "|" )

(or is it meant to be a ratio? W/H ? what's the point of that ?)

(what is vertical width? )

[+w<twist>] (degrees ? roll)

[+z<altitude> (unknown units earth-radii ? Km ? )

[r|R]|g] (still working on this one)



I have been poring over the docs but there is a dearth of examples with accompanying illustrations.





Documentation source:

http://gmt.soest.hawaii.edu/doc/latest/gmt.html



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I may have neglected to explain the project... sorry.

I am an historian working as an archivist but I also write code that amateur astronomy people use. I am upgrading scripts that convert JPL images to Plate-Carre format so they can be used in 3D image rendering apps. (PovRay, Blender, xPlanet, etc.) These scripts currently rely on MMPS, ImageMagick, Bash, and some other Linux commands. I want to use components that are more precise, cross platform, and are compatable with a variety of graphical user interfaces (probably wrapped in Python). Gdal is an option but seems to have a flaw in rendering a complete Equidistant Cylidrical projection from perspective or orthographic image sources. I am now exploring GMT's capabilities. I am not a G.I.S. professional so I am going to make many mistakes interpreting industry jargon and translating to traditional, imaging, or celestial terms.

I believe in the Feynman hypothisis that only the ability to teach a topic indicates true understanding, so I will be composing a tutorial of my own for the use by my intended audience. For this reason I may ask astonishingly naiive questions such as "Do ESRI numbers refer to ellipsoids and geoids and projections?" etc.

Although I may occasionally sound stu[pid, I am not. I am one of the forum moderators at xplanet and I wrote the TravellerWorldViewer web application. I know, not much.

anyway, that's why I'm here, why I'm involved, and why I will be asking questions as I zoom through the documentation.

Morfydd James (Molly)

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Like I mentioned in the other answer, please post a reproducible example. And two other things

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Thank you Joaquim,

I have been studying documentation from several sources including the official ones at this domain. I cited the U.Hawaii page as that one had the example.

I am restricted to version 6.3.0 as that is the one available to my inteded audience. They will be using the gmt package provided by the ubuntu/debian repositories.

My tests:

Molly’s GMT test: Input basic.png is 3600x1800 plate-carre Earth

gmt grdimage -JG+5/+50/20 -Rd basic.png -png map_JG-5
(Successful 20cm orthographic image centered on Cardiff ! Yay !)

gmt grdimage -JG+5/+50/20 +a30 +t5 +v1 +w0 +z5 -Rd basic.png -png map_JG-5

gmt [ERROR]: Cannot run a one-liner modern command within an existing modern mode session

Do I need to “Clear” something ?

From the man page:
-Jg|G lon0 / lat0 / scl | width [+a azimuth ] [+t tilt ] [+v vwidth / vheight ] [+w twist ] [+z altitude [r|R]|g] (General Perspective)

Question one -- Is it manditory that the perspective parameters be within a gmt script ? Is it impossible to output a specific view with one line of code?
Question two -- Does gmt have an inverse projection function? Can I reproject from ortho back to plate-carree ?

Thank you again for your patience. As I build up a set of working examples, I will be able to use them as templates for further trial and error study. This will be essential for the tutorial for my imaging scripts.
--Molly
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And no errors between first and second grdimage commands?

Yes. Need to do gmt clear sessions. But this is only necessary when a previous error left behind and non-finished session.

The old docs show that? New show

Syntax

-Jg|Glon0/lat0 /scale|width[+aazimuth][+ttilt][+vvwidth/vheight][+wtwist][+zaltitude[r|R]|g]

i.e., the GMT golden rule no spaces inside option strings

Yes, they are called one-liners and you used them (docs explain it)

The official answer is no. One need to make the tour proj1 → geog → proj2 but it’s actually not impossible that we can. Since PROJ created the pipelines we can go directly from proj1 -> proj2 and since we accept proj strings in GMT it is not impossible that it can be done, but I never tried. Note that here I’m talking only in image reprojections, not map-making that has different issues.

Now this is bad, really bad. See, we spend 3 years fixing and improving things and next users, not rarely, come asking for help for things that … have been fixed meanwhile. Manipulating images in GMT is one of those cases.

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Thank you Joaquim.


What does the correct command look like ?
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No spaces inside the -J option

gmt grdimage -JG5/50/20+a30+t5+v1+w0+z5 -Rd basic.png -png map_JG-5

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