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Misplaced images, objects and terrain meshes

 

 

If you get images with blue background (in the ocean) or with white background (in the north/south pole) or objects incorrectly placed when using some CAD-Earth commands the problem is due to an incorrect georeferenced drawing.

 

Before you can use some CAD-Earth commands you must georeference your drawing to define parameters like translation, rotation and scale to convert between drawing and latitude/longitude coordinates. This ensures that imported images and objects from the real world will be correctly placed in your drawing and when exported they will check with real geodesic coordinates.

 

You can georeference your drawing by any of the following methods:

 

By geolocating drawing objects in a map. This is the easiest method to georeference a drawing. You have to select some reference entities in your drawing (like lines or polylines) and locate them in a map. You can move, rotate and scale the reference objects until they are correctly placed. This is the least accurate method of georefence because the objects can only be approximately placed.

By selecting two points in the drawing and typing their latitude/longitude coordinates. If you know the corresponding geodesic coordinates of at least two points in your drawing the georeference parameters can be calculated with this information. The accuracy of the conversion calculations will depend on the precision of the latitude/longitude coordinates given.

By selecting a coordinate system. This is the more accurate way of converting between drawing and latitude/longitude coordinates, because all the conversion calculations are purely mathematical. You must know the coordinate system used when the drawing was made and the drawing coordinate extents must correspond with the real site, otherwise you'll have to almost certainly try several coordinate systems until the images and objects are correctly placed.

By loading georeference parameters from an existing file. If you have previously saved georeference parameters to a file from another drawing you can use the command to load georeference parameters from file to georeference a drawing that shares the same geolocation.

 

If you have georeferenced a drawing by selecting a coordinate system and get inconsistent results, the problem may be due to the following reasons:

 

Incorrect hemisphere (WGS84 coordinate system). The North hemisphere must be checked for sites located above the equatorial line and the South for sites located below. If the equatorial line crosses the site, first select the north hemisphere and then the south to see which one produces the most accurate results.

Incorrect units. Choose the units used when the drawing was created. If feet or meters units weren't used when the drawing was made, the entire drawing must be scaled by a conversion factor using the absolute drawing origin (0,0,0) as a base point.

Incorrect coordinate system. If you need to import images or objects from Google Earth to an existing drawing you must know the coordinate system used for conversion between latitude/longitude and XYZ drawing coordinates. If you don't know which coordinate system was used, please try the following:

 

If the site is located in the United States of America most probably the State Plane Coordinate System (SPCS) was used for the conversion. If this is the case, select 'SPCS (United States)' from the 'Filter by country' list in the 'Select grid dialog box' and choose the grid that most closely match the country sector (North, South, East, West, Central, North Central, South Central, East Central or West Central with the NAD83 or NAD27 datum) . It may be necessary to try several coordinate systems until the objects or images are correctly placed. Feet units and North hemisphere must also be selected for correct conversion in the 'Datum Settings' tab of the dialog box displayed when a CAD-Earth command is activated.

If the site is located elsewhere you can select the country in the 'Filter by country' list or type a word in the search box to narrow down the grid list, and select the coordinate system that most closely matches the site location. Make sure you select the correct hemisphere (north/south) and units (feet/meters).

If none of the above recommendations places the images or objects correctly, may be the world standard datum WGS84 or WGS72 was used for coordinate conversion. You can select the 'WGS84 (World)' from the 'Filter by country' in the 'Select grid' dialog box to display only WGS84 coordinates systems. If you don't know the UTM zone for the site but you know the approximate latitude/longitude coordinate values for the site, you can select the 'Calculate UTM Zone' buttons in the 'Datum settings' tab of the dialog box displayed when a CAD-Earth command is activated.

If you can manually locate the site in Google Earth, you can draw a polygon surrounding the site and then import it to AutoCAD using the CAD-Earth command to import objects from Google Earth with various coordinate system/hemisphere/units settings until the polygon matches your drawing. Then use this coordinate system for every CAD-Earth command to get consistent results.

In some cases, objects are placed within a few feet/meters from the correct position in Google Earth. This may  be due to difference between projection systems. Google Earth uses a cylindrical projection system. If you choose a coordinate system based in another projection (for example, conical) it may be required to adjust the position manually until the object is placed in the correct position. For this reason, in some CAD-Earth commands it is possible to specify offset distances in the X and Y direction before exporting images or objects to Google Earth.

 

If you can't still place the images or objects correctly please send us the DWG file and approximate site latitude/longitude values to support@cad-earth.com or submit a support ticket to help you find the correct coordinate system.

 

 

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