shrug-l: Web Mercator

Porter, William B. william.porter at famu.edu
Wed Jun 6 11:06:33 EDT 2012


After learning about the Web Mercator Projection  at the last SHRUG
meeting - and being puzzled why ESRI called it a projection [not a
simple variation on Decimal Degree],  I had an application for this
projection.   The Blackboard Corporation is creating a web applet for
FAMU, using map data I provided.  I had converted my precise, state
plane HARN map into  WGS_1984.  I gave them point [X,Y] data from the
same projection.  

 

The day after the SHRUG meeting I was told my DD map did not line up. I
guessed instantly that they were using Web Mercator.  I projected the
map to Web Mercator and they were happy. 

 

Then I was asked to create several more campus points:  Campus Center,
the Pool, Tennis Courts, other stuff. 

I worked with the data in Web Mercator,  then did a new "Add XY",
exported the data and didn't look closely at it.   I received complaints
from the Blackboard people that it was in a different format.  I checked
- and they were right.  Instead of -83.nnn, 30.nnn, it was  in the
format -93nnnn,  470nnn .  More like UTM coords.   After an unsuccessful
call to ESRI about it,  I Googled the alleged projection and discovered
this:  Web Mercator is used by Google, Bing and ESRI for world scale
mapping.  Here is the strange part:  It creates a hybrid [or
bastardized]  DD-Cartesian coordinate system,  outputting X,Y points in
meters.   ESRI has a 'simple' conversion showing how any point is
converted.  

 

So, Web Mercator is a 'projection' of sorts, but it has limitations you
may not expect, aside from low precision and distortion proportional to
distance from the equator [although the math tries to reduce that].  I
suppose it being called "Mercator" would have alerted a more clever
person, but I missed it. The ESRI tech clearly had no idea of the
relationship between DD and Web Mercator and no one had asked before.  

 

 

William  B. Porter     william.porter at famu.edu     850 412 5972

Application Coordinator for GIS / Infrastructure

Florida A&M University

EIT / GIS

Perry-Paige, West Wing Ground floor,

Office 1 -D

1740  S. M L King  BLVD

TLH FL 32307

 

 

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