[mapguide-users] Coordinate System Overrides

[nate_tate]

Hello fellow GIS, I'm new at using MG opensource and Maestro, so still
trying to get use to it. I would like to ask for some help or pointers. I
have an Oracle db setup and Maestro is connected to it and reads/see the
spatial tables. In the oracle db, the source SRID that is set up on the
spatial tables is 4326 and 8307, which is equivalent to each other.(see
screenshot).
<Loading Image...>

I use the edit tool to override the source 4326 and 8307 SRID into 3857(see
screenshot2).
<Loading Image...>

I am planning to use Openstreet map and or Google map as the base map and
overlay the spatial tables in Oracle on top of it. Hence the overrides to
use SRID 3857 which is the same as 900913 that OSM and Google map uses. I
guess my concern is that will this method work as far as the vector layer
from Oracle will fall/overlay on top of the base map correctly? Is this a
reasonable sound approach?

Thanks,




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[Jackie Ng]

Coordinate System Overrides is a slight misnomer.

What you're actually overriding is the declaration of what the coordinates
in OracleSrid4326 and OracleSrid8307 represent. You are not transforming
anything by changing the coordinate system here.

Coordinate System Overrides is you telling MapGuide: If you see data from
this Feature Source in OracleSrid4326, I want you to treat any data as
though it were in the coordinate system of <your selected coord sys WKT>

So to then answer your actual question. The coordinate system declared in
your Map Definition is the actual source of truth. Any layer -> feature
source whose coordinate system does not match the one in the map def is
automatically reprojected by MapGuide.

So what you really want (for your data to line up with OSM), is to have a
Map Definition in WGS84.PseudoMercator (EPSG:3857). MapGuide will reproject
any data from layers/featuresources in that map def that don't have the same
CS as the one you declared on the map def.

- Jackie



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[nate_tate]

Thank you Jackie.
What about if the Oracle SRID is already in 3857. If I transform the
SRID4326 into SRID3857 at the data base level, then it should fall/overlay
correctly?



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[Jackie Ng]

Yes, if the data is natively 3857 it should line up correctly.

- Jackie



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