Re: [MI-L] MapInfo Latitude/Longitude reprojection from generic to WGS84.

[Richard Greenwood]

On Wed, Jul 18, 2012 at 5:42 AM, Dan Jones <dan....@lunarfish.co.uk> wrote:
> Hello.
>
> I'm using MapInfo 10.0.1 (on Windows 7 Pro installed as 32bit app on 64bit
> OS if it makes any difference).
>
> I've got 2 layers; 1 is in the default LL projection "Latitude / Longitude"
> and the 2nd is saved as a copy with the WGS84 projection "Latitude /
> Longitude (WGS84) [EPSG:4326]".

In MapInfo the default "Latitude / Longitude" has not datum associated
with it. It is sort of a Camelleon and will "assume" the datum of the
map window (which in turn may be set by a layer that does have a
datumn). By saving a copy as WGS84 lat/long you have assigned a datum
to your layer, and possibly the wrong datum. Are you sure your data is
WGS84, not some other datum? EPSG:27700 is not WGS84, it is OSGB36. I
am guessing that the error you describe below is due to the difference
between WGS84 and OSGB36 and that your original lat/long is on OSGB36.

If I am correct then simply saving the original lat/long to British
National Grid may do the trick.

Rich

> If I open these in the same map by default they overlay perfectly but if I
> then change the map projection to British National Grid [EPSG:27700] (in
> order to use raster background data) the polygons separate. The originals
> still match up to the aerial photo background data but the WGS84 polygons
> have moved north and west and are slightly clockwise rotated. Can anyone
> explain what causes this and if there a way to resolve it? I thought that
> explicity re-projecting all the data into WGS84 would stop there being any
> re-projection issues rather than creating them.




--
Richard Greenwood
richard....@gmail.com
www.greenwoodmap.com

[college.atlas]

This can be a bit daunting when you start mixing map projections and rasters.

Firstly, Rasters take absolute precedence in map windows when it comes to map projection. You don't need to change the map projection of your mapper to match your raster. The raster will force this to occur. You also can't change the mapper projection if there is a raster in the mapper.

In addition if you have two rasters open in the same mapper, the raster that occupies more than 50% of the visible map window will be the dominant projection for the mapper

Another issue is that if you have rasters and they're not correctly registered you will get some very funny results in you vector data.

In Mapinfo if your raster is mis-registered Mapinfo can't warp or otherwise alter the shape of the image to fit your registration points. Instead vectors are warped and twisted to suit.

For example if your image was rotated off axis. Mapinfo will allow it to be registered but you'll have residual errors. If you then overlayed an object that is otherwise square, you would see it skewed like a parallogram to match the underlying raster.

In addition certain specific object types like Rectangle and Round rectangle don't take kindly to being forced into different projections. These tend to stay as rectangles and roughly translate rather than warp.

It sounds suspiciously like your image may be bit out of shape. This might be something as simple as the raster just not lining up with your data.

On Wednesday, 18 July 2012 21:42:03 UTC+10, Dan Jones wrote:

Hello.

I'm using MapInfo 10.0.1 (on Windows 7 Pro installed as 32bit app on 64bit OS if it makes any difference).

I've got 2 layers; 1 is in the default LL projection "Latitude / Longitude" and the 2nd is saved as a copy with the WGS84 projection "Latitude / Longitude (WGS84) [EPSG:4326]".

If I open these in the same map by default they overlay perfectly but if I then change the map projection to British National Grid [EPSG:27700] (in order to use raster background data) the polygons separate. The originals still match up to the aerial photo background data but the WGS84 polygons have moved north and west and are slightly clockwise rotated. Can anyone explain what causes this and if there a way to resolve it? I thought that explicity re-projecting all the data into WGS84 would stop there being any re-projection issues rather than creating them.
 

Thanks. Dan

[KTG Infotech]

lat/long to other projection is always creating mismatches as per my experiements, try and best of luck.

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[Cinda Graubard]

MapInfo's generic LL is a problem, always.

No other GIS software uses anything like it.  If you open a generic LL MI file in other software, it always 'changes' it to something else (LLWGS84 usually). Try it in Global Mapper for example. Or Arc.

Within MI we have found that the MI generic LL takes on the datum of the map window as it has none of its own.

Bottom line- you'll have to know or figure out the proper datum for your data and then create a version in LL(DATUM) (like LLWGS84) for example.
We had to replace a huge number of legacy files when we discovered how these actually behave.

It actually doesn't have to do with rasters, reprojection of rasters or vector data, or the like, in the map window. We tested it up one side and down the other.

Cinda Graubard
GeoMax

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