7 replies to this topic

[JacobL]

Hi there, this is my first time posting and I hope someone can help me out with an explanation so simple a caveman could do it.

I need to place a scale on a map that is a number of concentric circles around a point. So, the point is Guelph, Ontario, Canada, and there is a circle at a 100km radius that includes Toronto, Woodstock (ontario), durham and a bunch of other places that are all 100km in any direction from Guelph. THEN, there would be a circle 150 km from guelph including detroit, peterborough etc. THEN a 250 km circle etc etc etc, get it?

The trick is that I'm using a world map to include places far and wide such as argentina, south africa, and other fabulous destinations.

I have a Robinson projection as an Adobe Illustrator file, is there a simple way to draw a circle on the "surface"? Right now I am struggling through using the free transform tool with a circled-square, using the longitude-latitude lines as a guide.

Does this make any sense at all?

thanks, Jacob

[AndyM]

I know I've done this in the past in ArcGIS, just can't recall the details, and of course I'm not at work. Probably can be done in other GIS's too, but sounds like you are looking for geodesic buffers.

See this

http://blogs.esri.co...s-in-ArGIS.aspx

If you don't have access to software that will do this, someone may be able to help.

Or maybe someone has a more elegant, low-tech solution.

[JacobL]

Thanks for the info andy, i'll have a look at it right now.

since i posted this I found

http://www.freemapto...round-point.htm
and
http://www.gpsvisual...com/calculators

i can get a "distorted" circle from either of these sites and I thought I would take a screenshot and lay it over top of the vector map i already have. It's not pretty but maybe it will work. I'll post my results.

[sitesatlas]

You'll probably want to use an azimuthal equidistant projection centered on Guelph. That way, all the distances from that center point are equal and you will have perfect distance rings, much like a polar projection centered on the North Pole. If you draw a true circle and reproject it to Robinson or almost any other projection, the circle will be deformed into an ellipse (see http://en.wikipedia....nson_projection).

[JacobL]

what about using an unprojected map? Where the latitude and longitude lines are the same dimensions everywhere?

[Melita Kennedy]

what about using an unprojected map? Where the latitude and longitude lines are the same dimensions everywhere?

An 'unprojected' map is actually using a variant of the Plate Carree projection. A circle on a globe will be a circle in Plate Carree when centered on the equator and not very large. Away from the equator, it will be stretched east-west to reflect the fact that the longitude lines aren't converging at the poles.

Melita

[JacobL]

An 'unprojected' map is actually using a variant of the Plate Carree projection. A circle on a globe will be a circle in Plate Carree when centered on the equator and not very large. Away from the equator, it will be stretched east-west to reflect the fact that the longitude lines aren't converging at the poles.

Melita

thank you very much for your post Melita. That explains some problems i'm having then plotting distances. where's the flat earth society when you need them? back to the drawing board. blerg.

[M.Denil]

"... centered on the equator and not very large" should really read: "centred on the equator and infestesimally small".
The non-converging meridians, compounded by the fact that angular coordinate units (latitude and longitude) do not represent consistant measured distances on the ground, makes any shape anywhere on a plate caree suspect.