Celestia Forums

Ok, how do I figure out the sizes of landforms, oceans, or other geological features on other worlds, using their radii and their surface textures. I've no clue how to do that using the equirectangular projection Celestia uses.

How might I go about doing such calculations?

Maybe softwares like this (freeware) can help you. See the tutorials.
You can achieve landscapes like below and more for the usual projection:

Fenerit wrote:Maybe softwares like this (freeware) can help you. See the tutorials.
You can achieve landscapes like below and more for the usual projection:

No, not that. I meant the distances from Point A to Point B, and so forth, as well as figuring out how large land forms are, and how much of the planet's surface is ocean, on PRE-EXISTING maps.

Maybe some of the tools on "freemaptools" might help.
For example:
http://www.freemaptools.com/area-calculator.htm

selden wrote:Maybe some of the tools on "freemaptools" might help.
For example:
http://www.freemaptools.com/area-calculator.htm

Although I should've been more clear (mistake on my end), in that I'm looking for ways to do measurements for planets other than Earth (real and fictional), that actually looks like a good site for when I do Earth-based addons, so I bookmarked it. Thanks for the link.

If I've understood correctly, do you wish to measure both areas and perimeters of certain planets and/or moons in "advance", before whatelse terrain generator does build it by random once set forth its fictional radius. On which map do you accomplish such measures if the map isn't built yet? Thus, it is supposed that the future, fictional planet/moon, does have a radius of some sort and consequently a map already present of the known planets/moons which approximating the unknow ones. On the other side, if you wish just to know how big is a basin or how long is a ridge on Japetus, for example, you need a GIS program where you can geo-referencing its map accordingly with its radius/diameter and then making the measurements. Otherwise, do trace the area with the CMODtracer take apart its text's coordinates file and do import it within the above mentioned GIS program and do connect the points; then the program will find its area, the perimeter etc. IMHO, to build a map on crude measurements of such a sort is very tedious for a fictional object, being the scale in which the observer travel in the space well figured out by the fractal nature of the mountains and of coastals, for which to map a small moon with a texture suited for a planet wouldn't reveal the differences.

Fenerit wrote:If I've understood correctly, do you wish to measure both areas and perimeters of certain planets and/or moons in "advance", before whatelse terrain generator does build it by random once set forth its fictional radius. On which map do you accomplish such measures if the map isn't built yet? Thus, it is supposed that the future, fictional planet/moon, does have a radius of some sort and consequently a map already present of the known planets/moons which approximating the unknow ones. On the other side, if you wish just to know how big is a basin or how long is a ridge on Japetus, for example, you need a GIS program where you can geo-referencing its map accordingly with its radius/diameter and then making the measurements. Otherwise, do trace the area with the CMODtracer take apart its text's coordinates file and do import it within the above mentioned GIS program and do connect the points; then the program will find its area, the perimeter etc. IMHO, to build a map on crude measurements of such a sort is very tedious for a fictional object, being the scale in which the observer travel in the space well figured out by the fractal nature of the mountains and of coastals, for which to map a small moon with a texture suited for a planet wouldn't reveal the differences.

How are you not understanding this?

I'm talking about distances, measured in km or miles, across the surface of a fictional planet, from point a to point b. None of this random map generator nonsense. What I'm looking for is what some of the links lead to in the website Selden posted, but for fictional planets. I also want to find area in the same manner.

You need a GIS program. Or wait for a Google Bajor, a Google Romulus and a Google Mustafar.

If the distances you're interested in are the shortest possible distances between two points on a sphere, that's known as the "Great Circle Distance" and can be calculated from the latitude and longitude of the two end points and the radius of the sphere. It's also known as "the haversine formula".

See http://www.movable-type.co.uk/scripts/latlong.html


One way to measure the area of a region on a sphere is described at

http://www.mathworks.com/help/toolbox/map/f5-7224.html

I found these pages by using Google. Searching for
area region sphere
will point you to many others, some of which might be more usable.

If you haven't already, you should join the Cartographer's Guild forum at http://www.cartographersguild.com/

A search of that forum came up with a method for measuring the area of an arbitrary shape on a map by painting the area white, increasing contrast until everything else is black, and then getting a count of the white pixels. Some adjustments to the technique would be needed if the area is large compared to the size of the 2D map projection that you're using. (e.g. use MMPS to center the area you're interested in.)

See the thread at http://www.cartographersguild.com/archi ... -9246.html

selden wrote:If the distances you're interested in are the shortest possible distances between two points on a sphere, that's known as the "Great Circle Distance" and can be calculated from the latitude and longitude of the two end points and the radius of the sphere. It's also known as "the haversine formula".

FYI, for personal use I'd already enhanced the CMODtracer with such formula to get planets/moons' distance points, but i'm afraid that an update of the plugin shouldn't appreciated if is unclear about what we are speaking.

selden wrote:If you haven't already, you should join the Cartographer's Guild forum at http://www.cartographersguild.com/

A search of that forum came up with a method for measuring the area of an arbitrary shape on a map by painting the area white, increasing contrast until everything else is black, and then getting a count of the white pixels. Some adjustments to the technique would be needed if the area is large compared to the size of the 2D map projection that you're using. (e.g. use MMPS to center the area you're interested in.)

See the thread at http://www.cartographersguild.com/archi ... -9246.html

You may find this calculator useful if you decide to use an approach similar to that suggested by Selden: http://www.1728.org/sphere.htm
(I'm assuming that you want to derive the areas from the textures themselves.)
Assuming that you can count the appropriate pixels in a given horizontal line of a texture, and assuming your planets are close to spherical, then this calculator can be used to come up with a correction factor to be applied to account for the stretching effect as you get closer to the poles, allowing approximate areas to be calculated based on pixel count.

Near the bottom set the significant digits. 10 should be accurate enough.
Choose the second option "Sphere Radius 'r' & Cap Height 'h'", and then enter the planet's radius in the first box "Sphere Radius", and the texture line's distance from the pole in the "Cap Height 'h'" field.
Press CALCULATE.
You can then use the value from the "Spherical Cap Radius" field as a correction to your pixel count for that line of pixels.
Repeat for each vertical line of the texture.
I recommend entering 1 for the radius, and a fraction of that for "Cap Height 'h'", so that the calculated value can be simply multiplied by your pixel count.

If your texture is 2048 x 1024, then you'll have 1024 pixel counts representing the land area at each latitude, and 512 correction factors.
eg.
1. for the line of pixels nearest the equator with a land-surface pixel count of lets say 756 (out of 2048), we'd calculate the "Cap Height 'h'" as 511.5/512 = 0.9990234375.
So, enter 1 in "radius" and 0.9990234375 in "Cap Height 'h'", press CALCULATE to get the correction factor, which in this case is 0.9999995232.
2. For a line of pixels at 45deg lattitude with the same number of land pixels, you would enter 1 in "radius", and 255.5/512 = 0.4990234375 in the "Cap Height 'h'" field, which would give a correction factor of 0.86546. So multiplying 756 x 0.8654608505 = 654.288402978, which is the corrected area (in terms of equatorial pixels) for that latitude.
3. For the bottom or top lines (closest to the pole): enter 1, and 0.5/512 = 0.0009765625, giving a correction factor of 0.04418338291, and a corrected pixel count of 756 x 0.04418338291 = 33.40263747996 (equatorial pixels).
ie. in this example 756 texture pixels near the pole has the same actual area as 33.4 pixels at the equator.

That's just 3 examples (all assuming the same pixel count). You'll need to do this once for each line of the texture (and your raw pixel counts will differ at each "latitude"), however if you calculate all the correction factors first, and put these in a spreadsheet, you'll just need to plug in each of your pixel counts and get the spreadsheet to calculate the actual areas based on the radius of the planet ( using the corrected pixel counts of course).

CC

If you can get your planet texture showing on Google Earth, you can use its distance and area tools to calculate the distances and areas on your planet. All you have to know is the ratio of radii of Earth and the planet: ratio = radiusPlanet/radiusEarth.

In order to get the distance between two points on your planet, measure it with the Google measurement tool. Take the value shown and multiply it by ratio to get the actual distance.

For area, multiply by ratio^2.

Bingobangbongo.

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