Previously he had spoken of How to georeference maps in Google Earth, we will now see how we do the same with an orthophoto. Understand by an orthophoto, an orthorectified image, of which we know its georeference.
Google Earth requests four data, which correspond to the centers on each side of the orthophoto (see below image), many believe that the system has a margin of error since the orthophotos of a mosaic are not a grid whose edges coincide with a true north, rather they are segments of meridians and parallels.
However, there is no error in this, because Google Earth requests these four limits of the orthophoto in latitude and longitude format, so that it will be necessary to obtain all four.
In this case we are going to assume that we do it in Microstation.
Let us then go step by step:
To know the data of the image
1 Display the orthophoto in Microstation, for that you open a file, select raster manager / attach, disable the option place interactive and accept.
2. If you do not see the image, select it from the raster manager with the right mouse button and activate the option “fit raster to view”
3. Now, to know the coordinates, activate the tools/toolboxes/xyztext tool and use the “label coordinates” command
2. You need the coordinates of the central points of the orthophoto limits, so the best way is to make lines around it, now select the “label coordinates” command and click on the central points of the lines.
4. With this you already have the coordinates x minimum, x maximum, and minimum, and maximum.
5. By the way, export the image to formats that Google Earth supports, in my case it is in .ecw, for this only in the raster manager you select the image with the right mouse button and choose "save as", in this case the I will export to .tif format
Converting the UTM coordinates to Geographic
Although you can get them directly with others settings of microstation, we will illustrate this step always.
6. My location is in Honduras, my coordinates are as follows: X = 489885.60, Y = 1579986.30 X = 493260.30, Y = 1577678.70
As I need to transform them to latitudes and longitudes, I will use this site since our friend Gabriel Ortiz Has dropped its page. In this site I select the spheroid, in my case it is Clarke 1866, then the zone, which is the 16, the northern hemisphere (N) and enter the coordinate.
Since Google Earth asks me for the data in decimal format, I convert the seconds to decimal by dividing them by sixty, then the same by the minutes and the sums. The same procedure is done with the four coordinates, do not round because in this coordinate format a decimal of a second means a significant distance. You can also do this with a specialized program, there are many out there, it is not the subject of the post.
Importing the image into Google Earth
7 Open Google Earth and locate more or less in the country where the orthophoto will be; I suppose you had Google Earth closed because it consumes a lot of resources.
8. Select the option “Add image overlay“, you look for the image in the browser and give it a useful name for your purposes
9. In the last tab of that panel (location) You enter the four coordinates in decimal degrees formats, be careful not to erase the degree symbol.
10. Now try zooming in on the area, and you'll see the projected ortho; A good sign is that the box has the proportions of the image.
11. In the timeline, above the tabs of this panel you can select a transparency value, then the "accept" option and that's it.
In the next one, we will try the inverse, download an image of Google Earth and create georeference. Good luck and you let me know if I send you an invalid format message :)).