problem of wrong map distance

[Anonymous]

Hello, I use sasplanet software to download georeferenced raster images (*.jpg, *.ecw image format with georeferencing files : *.wld *.tab, *.dat, *.prj, *.xml, *.kml *.jpgw) ,it gives 3 different tupes of projection : EPSG 3395 , EPSG 3785 and EPSG 4326 The first problem is that EPSG 3785 doesn't figure in the coordinates systems library of autocad map . is there any equivalent projection ? The second problem is that when i insert raster images using MAPIINSERT command with EPSG 3395, the drawing coordinates are exact, but the distance measured on the map between two different points is wrong ,increased by 22% an this is equivalent to the latitude of 35°N (see attached image : 174m instead of 145m) . If i change the raster image scale when inserting it (using "modify correlation") , i get wrong coordinates , knowing that when i insert images taken from the equator 0°N , both coordiantes and distance are right. Please Help Thank you

[Anonymous]

attachement

[Alfred.NESWADBA]

Hi,

Hard to tell you anything without having the data, but what I see to that statement:

>> see attached image : 174m instead of 145m

The image shows me that you have not shown the second point for the command _MAPDIST ... what you are seeing on the screen is just what AutoCAD dynamic input shows you, and that does not have anything to do with _MAPDIST.

- alfred -

[Pointdump]

A,

Welcome to the Forum.

More information please on why the distances and coordinates are wrong. As for MAPIINSERT, I've never had much luck with it and don't trust it as far as I can throw it. Have you tried bringing in a Shapefile outline of your area of interest to compare?

As for EPSG 3785, according to the GeoRepository:
http://georepository.com/crs_3785/Popular-Visualisation-CRS-Mercator.html

"Relative to an ellipsoidal development errors of up to 800 metres in position and 0.7% in scale may arise. Some applications call this WGS 84. It is not a recognised geodetic system: see WGS 84 / World Mercator (CRS code 3395)"

4326 is the same as LL84 and is a Geographic Coordinate System. 3395 is a Projected Coordinate System.

Dave

[Anonymous]

Thank you all for your replies . Actually what is shown is the dynamic mode. when using MAPDIST , i get the exact distance(see attached  "map-distance" image). Here another exemple of "old trafford stadium" in Manchester , Lat is 54° N . The ratio between real distance using MAPDIST and ordinary distance using DIST command is 1/cos(54°) =1.67  ( For exemple the length of field : 105m with MAPDIST and 176m with Dist ) .Here is my question : everytime i want to draw a line (or any shape ) using a first point , a direction and a distance , have I to multiple the real distance by 1.67 to get the right shape ( to draw the centre circle of the stadium i have to take a radius of  15.3m instead of 9.15m , see attached "old trafford2" image), isn't there any other solution ?

[Alfred.NESWADBA]

Hi,

all that depends on the local geographic system you are working with. Every geographic system has a projection/transformation and depending on the system the difference between AutoCAD coordinate measurement and real GIS-measurement may differ more or less (whereas a factor of 1.76 is a lot).

So change the system to something better fitting to your local area and then you can work in most cases 1=1.

- alfred -