[Qgis-user] Projection of Shapefile export

[Brett Adams]

  Hi Roland,

Sorry but were straying well out of my depth here.

I vaguely understand the concept of what Proj4 and towgs84 do (but not 
really). I'll have to play with this idea.

Didn't know QGIS could do custom CRS definitions, although I'm not 
surprised.



On 21/09/2010 9:34 AM, Roland Hill wrote:
> I had problems a while back using AGD66, AGD84 and GDA94. Like you, 
> Brett, I wasn't getting the expected offset between data imported as 
> AGD66/84 and GDA94. I am going from a distant memory so just ignore 
> anything I say if it doesn't make sense, but there could be two issues 
> here.
>
> 1) I think Proj4 only defines a CRS if it doesn't have conflicting 
> definitions. There are multiple ways to define the towgs84 parameters 
> for AGD66 and AGD84, so Proj4 doesn't include any of them. If you look 
> at the Proj4 strings (in the QGIS CRS chooser) for say AGD66/Z55 and 
> AGD84/Z55 they are the same ( +proj=utm +zone=55 +south +ellps=aust_SA 
> +units=m +no_defs ). From memory, I think I got around this by 
> choosing a towgs84 definition that I liked and then just used that as 
> a custom CRS. You could use the custom CRS generated when importing a 
> MapInfo file for example.
>
> 2) About 6 months ago a problem with the custom database structure was 
> fixed. This was preventing custom CRS definitions from being used. If 
> you have been using QGIS for a while it might be worth deleting the 
> custom qgis.db (/.qgis/qgis.db in your home directory on Linux) so 
> that Qgis recreates it. This could be the reason layers are defaulting 
> to WGS84.
>
> Roland
>
>
> -- 
> *ROLAND HILL*
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> On 21/09/10 10:21, Brett Adams wrote:
>>  Enabling "On-the-fly projection" does make a small difference but 
>> the offset should be approx 195-200m. I get 12m.
>>
>>
>> On 21/09/2010 4:42 AM, Jürgen E. Fischer wrote:
>>> Hi Brett,
>>>
>>> On Mon, 20. Sep 2010 at 22:11:17 +0800, Brett Adams wrote:
>>>> Actually, this is worse than I thought. QGIS is not recognising 
>>>> AGD84 or
>>>> GDA94. The data comes in but isn't attributed a CRS so defaults to 
>>>> WGS84.
>>>> If you import the text file as AGD84, then again as GDA94, there 
>>>> should be
>>>> approx 195m separation. QGIS plots them in exactly the same place.
>>> But have have set a specific projection in your project properties 
>>> and enabled
>>> on-the-fly projection?  Otherwise QGIS won't reproject anything.
>>>
>>> Jürgen
>>>
>>
>>
>
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-- 
Brett Adams
Spinifex Geophysics
0438 861 974
SKYPE:brettadams_spinifex

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