[Qgis-user] WMS Image Projection

[kirk]

Hi Springer.I believe you will need to apply a grid shift file (NTv2).  It is available from Natural Resouces Canada and I will send you a link when I get to the office.The installation is a simple procedure. Kirk SchmidtSent from my Galaxy
-------- Original message --------From: Andreas Neumann <a.neumann at carto.net> Date: 2021-04-08  6:52 a.m.  (GMT-04:00) To: Springfield Harrison <stellargps at gmail.com> Cc: qgis-user <qgis-user at lists.osgeo.org> Subject: Re: [Qgis-user] WMS Image Projection 
Hi,
The approx 1.5m offset indicates that QGIS doesn't take a necessary grid-shift file into account. The grid-shift file is a separate file that defines the displacement vectors of coordinate systems with different datums.
As a European GIS user I am not comfortable with North-American CRS, but looking the two CRS up at epsg.io I can see that
http://epsg.io/?q=26910
vs
http://epsg.io/?q=3157
EPSG 26910 claims to have an accuracy of 4 m while EPSG 3157 claims to have an accuracy of 1m.
There might be a grid shift file necessary between the two CRS (I am personally unsure if that is the case here).
Since both, the project and the WMS are in the same CRS, I would assume that the offset problems occur with the parcel shapefile.
Do you know a local GIS expert in your region who knows if the transformation between EPSG 26910 and 3157 requires a grid-shift file? I would assume yes, because at epsg.io it indicates that the datums of the two CRS are slightly different: NAD83(CSRS) (http://epsg.io/4617) vs NAD83 (http://epsg.io/4269).
From there - and if we have a proper corresponding grid-shift file - we can help you set up the transformation in QGIS.
Greetings,
Andreas
On 2021-04-08 10:24, Springfield Harrison wrote:



Hi Andreas,
Thanks for the reply, I'll try to clarify:

Version is 3.16.4
The two municipal sources are being viewed in their own web windows and are displaying the same data

They are the source of the property lines for my GIS (downloaded shapefile) and the orthophoto (WMS link, not resident TIFF or other)


The property lines are EPSG:3157 - NAD83(CSRS) / UTM zone 10N - Projected, and from the municipal source.  I've attached the shapefile if you have the time.
In QGIS, the ortho is loaded from the WMS source

https://mapservices.crd.bc.ca/arcgis/services/OrthoImagery2019/MapServer/WMSserver
EPSG:26910 - NAD83 / UTM zone 10N - Projected


The project CRS is EPSG: 26910 but it doesn't seem to change anything by changing that or the source projections (at least within versions of NAD83 UTM Zone 10N)
Thus, the same information displays differently online versus in QGIS.

The shift seems to be 1.5 m +/-, quite a lot for what I'm trying to do (building permit with tight setbacks).
These are commonly used projections for our neck of the woods
The two images I sent earlier depict the differences; in QGIS, earlier orthophotos show a similar or worse displacement.  Not so on the municipal web viewer.


I've read up on projections in the tutorial and Help file, no revelations there, at least that I could see.

Thanks very much for any thoughts you may have.
-----
Cheers, Spring
 
 
On 08/Apr/2021 00:47, Andreas Neumann wrote:

Hi,
Are there two different CRS involved here? Are the vectors (parcel boundaries) in a different CRS than the orthoimages? This might be a problem of a missing grid-shift file in QGIS.
It is also unclear for me if you load your Orthoimages locally (from tiff-files as well) or from WMS (or both) and only one of them is shifted?
Can you also add information what QGIS version with what proj version you use? You can find this information in the about dialogue in the help menu.
Greetings,
Andreas
On 2021-04-08 02:37, Springfield Harrison wrote:

Hello All,Re obtaining orthophotos from a WMS server.Locally we have orthophotos that can be brought into QGIS using the WMS service.  They are also viewable on the local government web map service.  In both cases I can compare them to the property lines which are from the same source.On the government web pages (2), the orthophotos and the lot lines appear to coincide very well.  In QGIS, the same two layers are quite misaligned.One would suspect a CRS problem but changing the CRS of the Project and the WMS image in QGIS has no effect.  Even followed the tutorial to no effect.I'm attaching screen shots of the two results.  Ant thoughts would be appreciated, thank you . . . .-----Cheers, Springfield Harrison

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