Geographic Location

[squowse]

Hi, I gave this a go in AutoCAD 2014, just revisiting it in 2015 to see if it got any better.

What I don't understand is -

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If my drawing is in a known grid co-ordinate system (Ordnance survey UK) and I select this grid co-ordinate system under "geographic location" then why do I need to give it a geographic location point in both systems?

I have tried doing the conversion manually and giving it one point in both systems. I have also tried picking a feature and locating it this way.

Both ways the bing aerial photos only fit in one place - a few km away and the are significantly out.

[steve216586 · Accepted solution]

Perhaps:

There are three Norths commonly in use in Great Britain.

Grid North the direction of a grid line which is parallel to the central meridian on the National Grid.

True North is the direction of a meridian of longitude which converges on the North Pole.

Magnetic North is the direction indicated by a magnetic compass. Magnetic North moves slowly with a variable rate and currently is west of Grid North in Great Britain.

Differences

The horizontal angular difference between True North and Magnetic North is called MAGNETIC VARIATION or DECLINATION. The horizontal angular difference between Grid North and Magnetic North is called GRID MAGNETIC ANGLE. It is this angle which needs to be applied when converting between magnetic and grid bearings.

[squowse · Accepted solution]

yes that's spot on.

magnetic north is irrelevant for this but yes after I gave AutoCAD the lat, long and E,N of the point I needed to type in the meridian (true north) angle in the grid system. Using the spreadsheet here I calculated the "convergence" to be -0.127209164840828 at this point so changed this to positive (ie typed in 0.127209...) and now it looks a lot better! Within a metre or two at worst.

Thanks!

My original question (to the developers) does still stand though - AutoCAD has the co-ordinate system in it's database so why do I have to do the calcs for it??

[steve216586]

I think, (and may be corrected), but AutoCAD doesn't have a "world" coordinate system in the data base. It has a cartesian coordinate system in its data base, which can be applied towards world coordinates. Depending on where in the world you are, you will have to make calculations and considerations based on the true world coordinates vs cartesian system coordinates, which what cartographers, surveyors, GPS etc., use. The true coordinates that is, with delineation.

I should have added that is only to get a reference bearing location. After you find that, then all coordinates should be equal from AutoCAD's cartesian and world coordinates.

[squowse]

Yes AutoCAD works in cartesian co-ordinates but the geolocation function overlays aerial photos that are referenced by longitude/latitude.

One way to do that is to use an established grid system that has established parameters to project the cartesian co-ord system onto the curved surface of the earth. OS (Ordnance survey) is one of those, you can see there are many more in it's database (see attached screenshot). It just doesn't seem to want to do the calc itself!

Interestingly, although it needs a co-ord in both systems (one point translation) and a rotation it does seem to be able to calculate the scale factor itself.

The scale factor where the screenshot is located is 0.9996 for example, or 400mm in every kilometre, but I'm not seeing that distortion in the overlay it has done.