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Google Earth: Improving Accuracy

Google Earth images do not have photogrammetric accuracy.  The error at the centre of an image found by entering a latitude/longitude value in the stand-alone version of Google Earth and the correct position of that point may differ by more than ten meters even in areas of high resolution.  If you know the latitude and longitude of a trigonometric or other precisely known point which is visible in GE, you can confirm this by entering the lat/lon in the Google search box and examining where GE puts its centre point relative to the visible trigonometric point.  

In AirPhotoSE, use the Find in Coordinates for searching in GE with accuracy.  Do not use any other search option, and do not zoom into the image with mouse wheel.

Any known point used for searching in the Find by Coordinates search dialogue may be used, but accuracy differs.

1) Differential GPS or RTK  measurement in the field of a highly visible point in Google Earth with appropriate hardware: 

http://en.wikipedia.org/wiki/Real_Time_Kinematic

In Europe see:

http://www.essp-sas.eu/downloads/omsopvd/edas_sdd_in_force.pdf

2) A GE-visible trigonometric point with known coordinates,.

In the UK, you can obtain complete lists of latitudes and longitudes for more than 6000 trigonometric point pillars placed by the Ordnance Survey between 1936 and 1962 from:

http://www.haroldstreet.org.uk/waypoints.php?hill=trigpoints

Unfortunately, these points are no longer maintained and many are not  visible in Google Earth, but if one is visible in your immediate area of interest and in the same flight imagery,you can use the BNG coordinates EPSG 27700 in the Find in Grid dialogue box of AirPhotoSE.  This requires the use of the full OSGB coordinates in Easting and Northing. The map lettering scheme of British National Grid and the Irish Republic Grid must be converted to the long format.

Lists of trig points are sometimes available elsewhere, but many countries require payment for using them. The UK data gives a good idea of general GE accuracy under favourable conditions.

3) If you know the coordinates in the preferred local grid of any visible point from a GeoPortal site, you can also improve the accuracy of GE calibration in AirPhotoSE considerably by using the coordinates from the mouse position if the GeoPortal displays these to meter precision or better.

4) If nearby, the lat/lon coordinates of a navigation aid used for searching as described in the previous section of this documentation   The best points are close to major airports, and usually these areas may not be entered in a low-flying aircraft.  Be aware that the accuracy of these values is questionable outside of highly developed countries.

Warning:

Google imaging uses the Plate Carée projection which is a transverse cylindrical Mercator projection to a sphere and not to an ellipsoid.  The non-conformal Plate Carée projection is inherently distorted unlike most large scale maps which use a conformal projection.. For areas smaller than about 2 kilometres square, the error is usually of the order of a meter or two and can usually be neglected.  It becomes significant for larger areas, so try to zoom the Google image to enclose the smallest possible area required.  AirPhoto does not correct for  distortion in a Google Earth image but it does transform the calibration points at the corners and centre of the image to the ellipsoid of the UTM grid before applying a datum transformation to the chosen national or international grid.

When you download a Google Earth image using AirPhotoSE, 5 calibration points are written to a calibration file automatically.  All are invisible until you click on Hide/Show Calibration on the calibration menu.  Four of the points are near the corners of the image, and the fifth point with the label 9005 is easily seen.  This point will be used to correct the Google Earth image coordinates interactively.  All 5 points will then be moved to new positions and the centre point will lie over the point or object with known coordinates.  Depending on the scale of the Google Earth image after downloading as shown in pixels/meter on the status bar, careful correction can usually be +/- a pixel.  Depending on the precision of the known point, the remainder of the image will be geo-referenced with nearly this accuracy up to the limits of the Plate Carée distortion.  Since Google uses multiple images, often from different sources and dates, correction is not as accurate if different vertical image are captured in the area of interest. 

When a wide area is covered by the Google Earth image, the difference between the central calibration point and a known point can be considerable:



Additionally, when Google Earth images of multiple dates, as seen to the right of the image below, alignment in the stitched areas is not perfect.



Correction using the method described below can improve things, but high precision is not possible if the area is large.



GE images of different dates may not always be centred correctly and the central calibration point may be displaced.



In this case, the image with the displaced calibration point is the sharpest of the three.



This can be corrected as described below.  

Most trigonometric points are hard to find in Google Earth.  The navigation aids mentioned in the previous section are much easier to find and use for correction. 

Here are some step by step instructions.

Find a VOR-DME or VORTAC Navaid near an area of interest using http://www.ourairports.com/big-map.html

For example in Switzerland and the Zürich area:



Open the tab "Pilot Info" and scroll to the bottom of the page to find the NAVAIDS.



Zoom in to make sure that you can see the antenna:  Preferably choose one with "high power" if there are more than one available.



In AirPhotoSE, Google Earth, select "Find in Grid".  Make sure that you have chosen your desired grid in Options first.  Copy the coordinates to the Clipboard an paste them into the selection fields.



Select suitable height using the slider. Do not use the Zoom wheel.



Display the selected area in AirPhotoSE's Google Earth implementation.



Go back to the selection dialogue and change the altitude if more or less detail is desired.



Open the Calibration Menu and select Hide/Show Calibration to display the central calibration point.  It will usually be displaced from the true point.  This error is produced by a small possible coordinate error and by the errors due to manual placement of the image by Google.



Select the Move Cal Point option.  A magnifying window will appear when you move the mouse below the menu area.  Move this to cover the visible calibration point. 



Hold down the left mouse button and drag the point to the correct point in the image.

When you release the mouse button, the central calibration point will be moved to the correct position along with all the other invisible calibration points.

If you have a high resolution GeoPortal which covers the region of interest, you can usually obtain better accuracy by using the coordinates shown in the GeoPortal window.  You can capture these for more accurate searching.



Then use the Calibration menu again and the Move Cal Point option as before.



The central calibration point using the GeoPortal values has moved to the south, but it's distance from the centre hasn't changed much.



The result in Google Earth will be almost as accurate as the GeoPortal, with very small differences attributable to the resolution and hand-placement of the calibration move magnifier.