How to add a scale to any map projection

[howiet]

I cannot seem to work out how to add a scale to any map drawn in Geocart3, I am sure Geocart 2 showed scales although I have not opened it to see where and how. Could you please help me as I don't want to take a scale off a google map and try and match it up with a map constructed in Geocart.

howeit

[daan]

Hello, Howiet.

Actually, neither Geocart 2 nor Geocart 3 can add a scale bar to the map. Curiously, we have had few requests for it. I encourage you to put a request in over in the Wish List forum so that others can chime in. It would not be a terribly difficult piece of functionality to add.

Best regards,
— daan Strebe

[howiet]

Thanks Daan,

I will add this to the wish list.

Regards
howiet

[Atarimaster]

Actually, neither Geocart 2 nor Geocart 3 can add a scale bar to the map.

Oh, but it can!

Well, sort of. With a bit of work and under certain circumstances.
However, the image below shows not one, but six scale bars, with each of the red and green “subdivisions“ (don’t know if that’s the right term) representing 1000 km. And the image was generated using Geocart only, no other software involved.

demo-scalebars-small.jpg (41.63 KiB) Viewed 19762 times

Here’s a larger version of the image

For the moment, I’m not telling how it was done…
Although I guess it’s not too hard to figure it out.

Kind regards,
Tobias

[daan]

Well done!

[Atarimaster]

Thank you!

Of course, I’m using a metric line database that draws portions of small circles along the parallels.
Since Geocart wants geographic coordinates for the start, middle and end points, I had to calculate the length of a degree of longitude at various latitudes in kilometers (using the formula I found at Wikipedia) and then, the start and end points so that the subdivisions form a continuous scale bar.

I wrote a little script which does that and outputs the code for the metric line database, because writing the script was actually less work than calculating the coordinates manually for a single case (i.e. 6 scalebars with 5 subdivisions of 1000 km) and will spare a lot of work each time I create others.
The script currently is quite crude and doesn’t have a lot information how to use it, but in case somebody wants to take a look, here’s the current version. It already fixes some bad input values (e.g. latitudes that don’t exist) but probably, it will be possible to create metric line databases that do not work.

Notes:
– After importing the database in Geocart, go to the Line Styles dialogue and look for Coverage/ScaleBarSubEven and Coverage/ScaleBarSubOdd to set your preferred styles.
– In the coordinates, the script creates more decimals than you actually need unless you want to create a scale bar for dust particles or something …
– And of course, you’ll get nice scale bars only when you’ve got a projection with straight parallels.


btw, the basic idea was to visualize that a single scale bar on a map can be quite misleading. While maps that have a scale bar often show a note that it refers to the scale along the equator, I suspect that many map readers will think that the scale bar will be a little off in other places, which of course is not the case.

Kind regards,
Tobias

[daan]

Inventive, but a lot of work! And, of course, only applies to a cylindric or pseudocylindric projection.

I don’t have any strong ideas for how to make static scale bars meaningful on general world maps. Mostly I don’t have any ideas about how to make people comfortable about not having a scale bar. If the application removes the scale bar after zooming out far enough to make the scale bar meaningless, then people will be confused about its disappearance. If the application keeps the scale bar, people may feel like they’re not confused, but the reality is that they simply don’t realize they’re confused.
¯\_(ツ)_/¯

— daan

[Atarimaster]

but a lot of work!

Ah well, the page I’ve linked to is about 10 KB, with half of it being the HTML/CSS code to put out the page, and the other half being the PHP code which is doing the actual calculations. That wasn’t that much work.

And, of course, only applies to a cylindric or pseudocylindric projection.

Yes, that’s what I meant with “under certain circumstances”.

I don’t have any strong ideas for how to make static scale bars meaningful on general world maps.

Me neither.
As I’ve said, I explicitly wanted to visualize what’s the problem with scale bars. I surely wouldn’t want to use it on a general world map, it’s really just a bit of playing around.