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