[Qgis-user] Problem georeferencing images in UTM

[0x906]

The UTM Easting and Northing are the following:

X = a*L ,
Y = a * tan^-1(sin(F)) - a*e * tan^-1(e*sin(F))

For the Web Mercator the same equations are:

X = a*L,
Y = a * tan^-1(sin(F))

What's going on here?  The Web Mercator as a projection is treating the 
Earth as being a perfect sphere.  On the other hand, the UTM is making 
use of the Earth's flattening value "e".  As you can see at the Web 
Mercator Northing equation, the flattening is zero (e=0) and thus, only 
the first part of the Northing equation produces a value.

[Sources: "Implementation Practice Web Mercator Map Projection" 
http://earth-info.nga.mil/GandG/wgs84/web_mercator 
/(U)%20NGA_SIG_0011_1.0.0_WEBMERC.pdf]




On 5/19/2018 3:32 AM, Andre Joost wrote:
> Am 18.05.18 um 22:30 schrieb Nicolas Cadieux:
>> Hi,
>>
>> NAD27 does touch the northern part of South America but you are correct
>> in saying that it does not extend in Brazil.
>
> Note that +datum=NAD27 redirects to the US/Canadian grid shift files 
> which do not even cover Central America.
>
> See 
> https://gis.stackexchange.com/questions/124322/reprojecting-between-nad27-and-wgs-84-part-iii 
> for a visualization.
>
> Greetings,
> Andre Joost
>
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