[Qgis-user] projection for equal distance in all directions, not equal angle

[Brent Wood]

You want projections 101 :-)

& don't ask a geodicist if you want an answer you can understand!

Any mapping of a spheroid to a flat surface involves a distortion. There are a few possible distortions:

shape
size
distance
direction
   (enough for now)

Depending on what you want to show, you should pick a projection to minimise that distortion.

an equal angle projection will tend to distort shapes least (angles are maintained, sometimes at the expense of sizes of objects or apparent distances
an equal area projection will distort relative sizes least, but shapes & distances can be warped
an equidistant projection will retain relative distances.. 
an azimuthal projection maintains direction (at least between a pair of points)

distortions are lowest at
 the center of the projection, and typically increase the further you get from the center. Any global map obviously has points as far from the center as possible, so involves significant distortions.


For a light hearted start on projections, try: http://xkcd.com/977/

For something a bit more informative:
Wikipedia: http://en.wikipedia.org/wiki/Map_projection
From the GMT Open Source cartographic suite: http://gmt.soest.hawaii.edu/doc/5.1.1/GMT_Docs.html#gmt-map-projections
 (while this does focus on using GMT to plot maps in various projections, it is a good primer - & does highlight the limitations of other GIS tools in this area)
And this from Colorado University is a good starting point: http://www.colorado.edu/geography/gcraft/notes/mapproj/mapproj_f.html 

HTH,

   Brent Wood


----- Original Message -----
From: Dave Kimble <dave.kimble at riseup.net>
To: qgis-user at lists.osgeo.org
Cc: 
Sent: Sunday, April 20, 2014 4:41 PM
Subject: [Qgis-user] projection for equal distance in all directions,	not equal angle

Up till now I have only looked at small areas, so this problem may not 
have been apparent, but now, looking at a "World" shapefile, the screen 
image is obviously distorted.

View > Decorations > Grid > X=1 Y=1
gives a square grid pattern, when I was expecting a "rectangular" 
pattern - reflecting the length of a degree of Latitude (constant at 60 
nautical miles = 111 Km) and the length of a degree of Longitude 
(variable - shorter near poles).

I
 have tried various different CRS selections, and 3 different 
shapefiles, but always the same - the screen image appears stretched in 
the E-W direction.

Plug-in "Globe" correctly maps the shapefile onto a globe, so it's not 
file at fault.

I'm at a loss for vocabulary to describe it properly, or to look it up 
in Help.
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