PDA

View Full Version : graphic: Earth lat/long from poles at a different position



DaveC426913
2016-Nov-27, 07:15 AM
This is for a fictional project, but the tools I need will more likely be found here in this forum, I think. Move to a "fiction" if necessary.

I want to render a map of Earth with the "poles" of the grid in a different place. Say, the South "pole" of the grid is at Tierra del Fuego in S. America. The North "pole" of the grid naturally will be at its antipodes. A Great Circle will define all points equistant from these poles - a tilted equator.

Pretend the Earth's axis (geo or magnetic) runs through these two points. Then draw new latitudes and logitudes.

(What I'm really after is not about the actual axis of rotation. I'm just after a crude grid that shows the distance to these points (erstwhile latitude lines) and the direction to these points (erstwhile longitude lines).)

I've started by using Google maps to "measure distance" from point to point, which gives me partial Great Circles. These make crude "longitudinal" lines, but now I want crude "latitude" lines (which are essentially all points equidistant from the poles).

Google maps is Mercator Projection-like so the lines form sine-like curves - very difficult to manually draw in latitudes.

Can anyone suggest how I might do this with only a modicum of effort? Any online tools that might allow me to draw poles and lines on a map of Earth?

Solfe
2016-Nov-28, 11:53 AM
I would think that you'd need two maps, one with lines drawn and one without and use GIMP's magic wand to select just the Mercator Projection lines and rotate them as needed. Since they would push off the canvas, you'd need to redraw parts of them using the line tool.

The problem with it is the projection itself is distorting the map one way, but the lines indicate a different distortion. I am not even sure if your antipodes would match.

Torsten
2016-Nov-28, 10:22 PM
Hi Dave,
I don't know about online tools, but I have software (Global Mapper) that allows me to add "range rings" to a map. So I set a point somewhere in Tierra del Fuego and added rings at 600 nm intervals to the World Street Map global view, which is in Mercator projection. I then exported these as a KMZ and opened it in Google Earth. Attached results below. Is this what you're looking for?

CJSF
2016-Nov-28, 11:57 PM
Hi Dave,
I don't know about online tools, but I have software (Global Mapper) that allows me to add "range rings" to a map. So I set a point somewhere in Tierra del Fuego and added rings at 600 nm intervals to the World Street Map global view, which is in Mercator projection. I then exported these as a KMZ and opened it in Google Earth. Attached results below. Is this what you're looking for?

Ooh! I work with Global Mapper just about every day. Hobbyist or geospatial career (or both)?

CJSF

Torsten
2016-Nov-29, 01:13 AM
Ooh! I work with Global Mapper just about every day. Hobbyist or geospatial career (or both)?

CJSF

It was a really useful tool in my forestry career, both for operational planning and doing research. I thought the learning curve was pretty easy, though I wouldn't describe myself as particularly proficient with it. Its ability to export both vector and raster data in numerous different formats was something I really appreciated. But it's mostly a hobby these days, and I'm not buying the updates.

I see you're a geographer/image analyst, so this software must be really useful to you too. And now I know who to ask for help!

DaveC426913
2016-Nov-29, 04:20 AM
Someone on another forum introduced me to this little beauty: http://ns6t.net/azimuth/azimuth.html

It allowed me to produce this:
21876

The straight radial lines are centred on my point of interest (Tunguska ground zero).
The disorted red lines are geographical longitude lines.

I could post a larger map and explain it little if anyone's curious.

Below is a closeup.

The compass rose is NOT magnetic North; it is True Geo North.


21877

Torsten
2016-Nov-29, 05:13 AM
Very nice! I had to look up the definition for that projection. Now I understand what you were after.

I also learned that the map in the UN logo uses that projection, centred on the north pole.

DaveC426913
2016-Nov-30, 03:19 AM
Very nice! I had to look up the definition for that projection. Now I understand what you were after.

It wasn't exactly what I was after; but it did solve the problem.

I wanted to be able to - with only a modicum of accuracy - be able to pick a location on Earth and be able to identify fairly easily what the true direction was to the Tunguska Site - both in terms of surface direction and declination (through the Earth).

(This is for a sci-fi game)

I could have done it on a flat Mercator projection, but not without a lot of difficulty plotting a second grid (whch would be all siney-wavey).

The big advantange that generator has - despite any downsides - is that it facilitates plotting a second grid on the Earth's sphere while retaining the original lat long grid.
And that means that mapping one set of coords onto the other is as easy as looking at the map.

No other solution I've found explored does that.