Jump to content

 
Photo

Projection Choice

- - - - -

  • Please log in to reply
6 replies to this topic

#1
Tomo

Tomo

    Newbie

  • New Member
  • Pip
  • 3 posts
  • No Country Selected

I'm working with a dataset in ArcGIS 10.1 that has an approximate range of 650 km N-S, and 400 km E-W, across southern BC, Canada and northern WA, USA. I need to find a projection that will conserve distance information within this range, as I will be doing analysis/calculations regarding distances. I had recieved some advice to make a custom projection, starting with the NAD 1983 BC Environment Albers projection. So I did this by setting the central median to the midpoint of my longitudes and the two standard latitudes to a bit below (1/6 of total spread) my northmost latitude and above my southmost latitude. The data is originally unprojected, with a Geographic Coordinate System of WGS84, so I changed the projection's GCS from NAD83 to WGS84, as well, to avoid transformations of lat/long.

 

One of my primary concerns with the projection is that Albers is equal-area, whereas I had originally assumed that an equidistant projection must be the best choice. Equidistant projections tend to only conserve distance information from one point, though, and I will need to look at distance information between a number of arbitrary points, so I now feel like equidistant may not be the best. As well, Albers is conic, which I have read may be better for data that is more spread out E-W than N-S, like mine is, but the alternatives to Albers that have been suggested are usually a UTM projection, which is a conformal projection. As I understand it, this would be a bad choice of projection for doing most kinds of measurement/analysis.

 

Given this information, can someone tell me if I have made an appropriate choice of projection? Any feedback/comment is much appreciated! Let me know if any additional information would help.

 

 



#2
Strebe

Strebe

    Key Contributor

  • Validated Member
  • PipPipPip
  • 87 posts
  • Gender:Male
  • Interests:Map projections. Snobby chocolate. Science in general.
  • United States

No projection will give perfect results. Why not just compute distances directly?

What are your accuracy requirements? Given the small region you're working with, the (not so very) custom projection you have may be quite adequate.

--daan Strebe

#3
Tomo

Tomo

    Newbie

  • New Member
  • Pip
  • 3 posts
  • No Country Selected

What do you mean by compute the distances directly?

 

I was not given the accuracy requirements, but I want to get as accurate a projection as I can.



#4
Strebe

Strebe

    Key Contributor

  • Validated Member
  • PipPipPip
  • 87 posts
  • Gender:Male
  • Interests:Map projections. Snobby chocolate. Science in general.
  • United States

Given two geodetic coordinates, you can compute the distance between them. There are libraries for that. It's not terribly difficult for distances that short.

--daan Strebe

#5
Tomo

Tomo

    Newbie

  • New Member
  • Pip
  • 3 posts
  • No Country Selected

Interesting, I may look that up. Thanks for the advice.



#6
Dennis McClendon

Dennis McClendon

    Hall of Fame

  • Validated Member
  • PipPipPipPipPipPipPip
  • 1,084 posts
  • Gender:Male
  • Location:Chicago
  • Interests:map design, large-scale maps of cities
  • United States

A search for

great circle distance formula

will yield a number of discussions, including this helpful blog post.


Dennis McClendon, Chicago CartoGraphics
chicagocarto.com

#7
Strebe

Strebe

    Key Contributor

  • Validated Member
  • PipPipPip
  • 87 posts
  • Gender:Male
  • Interests:Map projections. Snobby chocolate. Science in general.
  • United States

Or look up Vincenty's formulae for greater accuracy on the ellipsoid.

--daan Strebe




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users

-->