Google Earth Image question

[Tad_Thurston]

In Civil 3D, under the Insert tab, there is a menu selection that allows you to insert a Google Earth image directly from Google Earth. This image is inserted to match the coordinates of your ACAD drawing. However, it does not always match the known correct line work, depending on the drawing settings. For example, if one was working on a project in California in Cal. Coordinate System, zone 2, the available coordinate systems (under the Units and Zone tab) could be HARN, NAD27 or NAD83. Then there is the transformation settings (under the Transformation tab) like: Sea Level Scale Factor and Grid Scale Factor.

My question is: What are the best drawing settings to use for the best fit between the Google image and the drawing so that the image matches linework that you know is correct (based on certified surveys)?

Thanks.

[irneb]

AFAIK Google's images are set to GPS coordinates (i.e. Lon/Lat degrees). So depending on your target coordinate system it might be scaled / rotated / skewed / tilted any number of ways and any combination of those. Your best bet is to find the formulas for your particular coordinate system from GPS.

[Tad_Thurston]

irneb,

Thank you for the help. So I am thinking that Google uses to center of the image you have picked on your screen and uses that to locate the image in the ACAD drawing? And if that is true, how would I find out what those center coordinates are?

[Tad_Thurston]

irneb,

Sorry first reply had typo. Should read:

 

Thank you for the help. So I am thinking that Google uses the center of the image you have picked on your screen while in Google Earth and uses that to locate the image in the ACAD drawing? And if that is true, how would I find out what those center coordinates are?

[irneb]

I'm not sure if the coordinates are saved inside the "image" you got from GE. It probably depends on how you got that, if you've got GE Pro and used its save function, then it might have tagged the JPG/TIF with the GPS coordinates using EXIF. Otherwise (e.g. screen capture) it's highly unlikely - so you'd need to check the GE image at the bottom to see the GPS coords displayed.

 

I'm guessing the centre of the image would be the coords intersection yes. Though in most cases I adjust manually by scaling and rotating the image to match some measured feature of the area in question. I usually use something like road curb-lines (as many as possible) then try to match them as close as I can. So generally I don't use these coordination conversions much.

[SLW210]

You also need to realize that most of (if not all) Google Images have some distortion from not being perfectly overhead.

 

I pretty much adjust similar to irneb's methods.

[Tad_Thurston]

Thanks guys. I will look into the EXIF you suggest. I do not have Google Earth Pro at my desk but we have it in the office.

 

Viva La Cadtutor!