Google Earth Coordinates Don't Match Up

[jeffj]

Hi everybody,

I'm new to this CAD tutor site, and i'm also new to autoCAD. However, I do have experience with ArcGIS.

I really hope somebody can help me with this issue that I have!

I am attempting to map some lat long coordinates of several polygon centroids that I have created from a CAD map into Google Earth. However, after inputting a lat long coordintate into Google Earth, it is several metres off. For example, if the centroid is supposed to be of a building, on google earth it will show up on the street next to the building instead of on the centroid of the building.

Thanks in advance!
-Jeff.

[rustysilo]

Can you explain how you have gotten from point "a" to point "b" thus far?

[jeffj]

Sure, no problem.

I have a CAD map of polygons that outline buildings and roads.

I used the create -> centroids function to create dots where the centroids of the polygons are.

I then simply used the mouse to move over the centroid to get the coordinates of the centroids.

For example, one of the centroids is 43.6451, -79.3692 which is the centroid of a polygon representing a building.

The problem is when I enter the coordinates of 43.6451, -79.3692 into Google Earth. It shows the correct building, but is not very accurate. It seems that all the coordinates are off by several metres. Another example is that the coordinates of a centroid of a street in CAD shows up in Google Earth not on the street...

Any help would be sooooo appreciated, and please note that I'm definitely not an experienced CAD user.

Thanks,
Jeff.

[rustysilo]

What coordinate zone is your .dwg set up in? Is it set up properly and your cad objects located properly within said coordinate zone?

[jeffj]

What coordinate zone is your .dwg set up in? Is it set up properly and your cad objects located properly within said coordinate zone?

The Coordinate System is Ontario MTM Zone 10, and the objects should be located properly.

Do you think the difference in projection system plays a role?

[rustysilo]

I'm not sure exactly, but if your objects are all off by the same amount it makes me think maybe they aren't positioned properly or perhaps there's some adjusting needed in the process to bring it into GE.
Maybe you just need to "nudge" it to get the items where they should be?

disclaimer: I'm not a Map 3d guru or anything.

[jeffj]

I'm not sure exactly, but if your objects are all off by the same amount it makes me think maybe they aren't positioned properly or perhaps there's some adjusting needed in the process to bring it into GE.
Maybe you just need to "nudge" it to get the items where they should be?

disclaimer: I'm not a Map 3d guru or anything.

That makes sense, thanks for your help!

[eldon]

I am attempting to map some lat long coordinates of several polygon centroids that I have created from a CAD map into Google Earth. However, after inputting a lat long coordintate into Google Earth, it is several metres off. For example, if the centroid is supposed to be of a building, on google earth it will show up on the street next to the building instead of on the centroid of the building.


Are you perhaps mixing two entirely different systems of describing a location?

Latitude is an angular measurement in degrees, and a line of latitude is actually curved. You are trying to deal with this in a system of flat lines. I didn't know Map 3d was that clever :shock:

[jeffj]

Are you perhaps mixing two entirely different systems of describing a location?

Latitude is an angular measurement in degrees, and a line of latitude is actually curved. You are trying to deal with this in a system of flat lines. I didn't know Map 3d was that clever :shock:

I'm not sure I understand.. sorry, all I know is that the latitude, longitude coordinates from the autocad map doesn't correspond with the latitude, longitude on google earth.

I'm probably articulating my problem very poorly.. and for that I apologize.

I'm leaning towards the previous reply where rustysilo suggested that the objects are not positioned properly. I was not the one who created the drawing so I will confirm with the person who did.

[rustysilo]

If the dwg was set up in a coordinate system and the objects placed properly it should come into GE correctly using the Drawing Coordinate System Transform option. Have you gave that a go to see what happens or have you only tried the User Defined Transform?

If not then I guess you'd just use the Nudge option to "fine tune" the position of the objects.

[jeffj]

If the dwg was set up in a coordinate system and the objects placed properly it should come into GE correctly using the Drawing Coordinate System Transform option. Have you gave that a go to see what happens or have you only tried the User Defined Transform?

If not then I guess you'd just use the Nudge option to "fine tune" the position of the objects.


Okay, so the weird thing is that I cross-referenced the lat-long vertices of the polygons with the lat-longs in GE and they are the exact same. For example, the vertex of the polygon for the building in CAD matches the actual building corner in GE.

But the problem is with the centroids. They aren't where they're supposed to be.

[rustysilo]

So only your centroids are off?

[jeffj]

So only your centroids are off?

Exactly! Isn't that weird!?

[eldon]

Did you work out the centroids using plane geometry or spherical geometry?

[jeffj]

Did you work out the centroids using plane geometry or spherical geometry?

I'm not sure, I just used the Create --> Centroids function.

[rustysilo]

Yeah I think so. No idea what's causing that. Upload the file (if it isn't too big) and let me see if it does the same thing.

[jeffj]

Yeah I think so. No idea what's causing that. Upload the file (if it isn't too big) and let me see if it does the same thing.

It's in my best interests to upload the file for you to help (and thanks for offering!), but unfortunately I'm not allowed to because of company reg's. I'm going to have to talk to my boss about this, hopefully he will be able to help me out!

I really appreciate your help!

[CadBrat]

Hi, Jeff et al. Hope you got your alignment straightened out. I also have a GIS background but am pretty much new to CAD.

I've got the lower right corner of a building lined up in aCAD (dwg & raster) & I'm trying to shrink the raster by scaling it as an object. After I specify base pt. of the building upper left, I try to pull the rasterized corner to the dwg corner, but the raster will only Increase in size, not Shrink. Seems totally basic but I can't get it to work. FYI, the yellow arrays are proposed solar panels. (snapshot attached)Thanks very much in advance. -Michael

[chulse]

Hi, Jeff et al. Hope you got your alignment straightened out. I also have a GIS background but am pretty much new to CAD.

I've got the lower right corner of a building lined up in aCAD (dwg & raster) & I'm trying to shrink the raster by scaling it as an object. After I specify base pt. of the building upper left, I try to pull the rasterized corner to the dwg corner, but the raster will only Increase in size, not Shrink. Seems totally basic but I can't get it to work. FYI, the yellow arrays are proposed solar panels. (snapshot attached)Thanks very much in advance. -Michael

Try ALIGN...

[rustysilo]

I use Map > Tools > transform or rubbersheet in cases where I need to match an aerial (without a world file) manually to my dwg.

[zars]

You sure you're using the same DATUM? I believe that Google Earth uses WGS84.

[jeffj]

Does the difference in DATUM matter? If so, how do I change the DATUM in CAD to match that of Google Earth?

[eldon]

I am wondering if your apparent problem is anything to do with matching datums.

When you work in Latitude and Longitude, the distance in metres for one degree alters for Latitude as you go North. At the Equator, one degree is 111.2km for both Longitude and Latitude. At 44N, one degree in Longitude is still 111.2km, but one degree in Latitude is 79.99km. Does the centroid function allow for the different scaling in the horizontal and vertical directions? :?

[CadBrat]

wow- great string! 1st, i did resolve my immediate SCALE conundrum & finally figgered out that i needed to use ALIGN. whew! but maybe Datum was the underlying prob here. yup, gEarth default is WGS84. now i'm not in cadMap, so it didn't occur to me that Datum would be an issue in regular aCAD. im surpised that aCAD processes Datum (has a default?). according to rustysilo's comment, you can set up a dwg in aCAD (not only aMAP) with a coordinate system. i will check out the Drawing Coordinate System Transform & the User Defined Transform. But if i unnerstan correctly, some of jeff's linework is On but all of his centroids are Off. that might Not be a datum thing...? - Michael