Import Google Earth image into ecw format

The need: We need to work a cadastre using the Google Earth image in a georeferenced format that is lightweight.

The problem: The ortho that Stitchmaps downloads is in jpg format, the georeference that it brings is not supported by Microstation.

The solution: Download the image with Stitchmaps, synchronize Google Earth with Microstation to import the georeferenced capture and warp one against the other.

We are interested in ecw because it does not occupy an additional georeference file and where a 200 MB HMR or Tiff could weigh just 12 MB without losing much in quality. We have Stitchmaps and Microstation PowerMap V8i, as we have we will do it with this although with other programs it could be done with fewer steps.

Let's see how it is done:

1. The image download. 

This we have done with Stitchmaps, as already explained before. With the exception that we have drawn a rectangle in Google Earth, so that it is inserted in the capture of the images.

In Google Earth this is done with Add> Polygon, and in style we choose outline with a line thickness of 1.4 white. We will do this like this, because Microstation cannot import a kml file in these versions, except with FME from Bentley Map. But the version Powermap does not bring this functionality, so to create the rectangle we will have to do it drawing on the image.

2. Create a georeferenced dgn.

This is created by doing File> New, and we choose a Seed3D seed. Google Earth image import does not work on a 2D file.

Then we must add georeference to the file, that is done with: Tools> Geographics> Select Coordinate System

In the panel we choose From Library, and as this time we are interested in UTM zone 16 North, then we choose:

Library> Projected> World (UTM)> WGS84> UTM84-16N

If this is the system we use the most, we can right-click and add to favorites, to be able to access more easily. We make OK and our file is now georeferenced.

3. Capture image from Google Earth

To synchronize Microstation with Google Earth we do Tools> Geographics> Follow Google Earth View. In this way, our view is reflecting just what is on Google Earth. It is convenient to have oriented the north there and an acceptable approach.

To import the image we do Tools> Geographics> Capture Google Earth Image, we click on the screen and then complete deployment. What we have there is not an image, but a digital terrain model, with the image as property of footwear.

To see the image, we run the rendering. In order not to complicate where the render buttons are, I will execute it through the text command. Utilities> Key in> render all smooth.  See that there is the box that interests us. This image, despite its poor resolution, is georeferenced.

4. Georeference the image

For this, first, we will make points in the corners of the georeferenced image. This is done with the command of points, we will do them in green, with a representative thickness and with a suitable approach such that the corner of the rectangle is visible. If we lose the image, we execute the render command again, and we do not worry about being so exact, we remember that the accuracy of Google Earth is worse than what we could lose here.

Once the points are made, insert the jpg image that we have downloaded with Stitchmaps:  File> Raster Manager, then in the panel we choose File> attach> Raster. Let's not forget to leave the option active Place Interactively, because we will enter it manually.

We place it inside the box of the gray image, so that we can stretch it from there. 

Similarly, we make points to the corners of the rectangle that is in the color image. We will do these in red to notice the difference.

Finally we should have something like this:

To stretch the image, from the Raster Manager panel, we right click on the image, we choose Warp, with the method Affine of more than 3 points. Then we select each corner, indicating the point of origin (red) to the destination point (green) and when all four are there, we right-click the mouse.

5. Convert the image from jpg to ecw

Done, now our jpg image is georeferenced. To save it in another format, select it, click the right mouse button and choose save as. We can choose from many formats, including the precious Ecw that they did not have the Microstation versions. 

And finally we have what we needed, a size 24 MB raster, with a box of our interest of 1225 meters per side, ready to work.