Tracing Google Earth images

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Google Earth gives us convenient and free access to high resolution satelite images. These images are very useful as mapping reference.

Offset Error in Google Earth[edit]

Earlier Google Earth high resolution images have offset errors. Compare the images below:

GE image with offset error:

GE image with offset error corrected:

Determining Offset[edit]

Determining whether each GE high-res image has offset error and what is the offset is an essential step if you intend to make use of GE images.

  1. In order to determine the offset, you need to have some accurate tracklogs or a map drawn using accurate tracklogs.
  2. Narrow roads such as residential streets are good places to compare your trackogs against the GE image as narrow roads have less margin of error.
  3. Try to use an area where the roads run directly north-south and east-west. The pictures above showing the offset errors are good examples. From it, we can determine that the offset is only to the west. There is no offset error along the N-S axis.

Direct Trace within GPSMapedit[edit]

If you are willing to buy a copy of GPSMapedit, you can make use of its GE display feature to enhance your map. Tracing directly within GPSMapedit is least time consuming and most accurate (no calibration error). The drawback is the USD65 price tag but it is worth the money if you are a serious mapper.

Click on the Show Google Maps button to display Google Earth image at the background of your map.

  • You can draw objects by tracing the GE image at the background
  • If the GE image at that area is known to have an offset, calibrate the image by right-clicking the background and select Shift. Enter the calibration value to fix the offset.

Tracing using Google Earth and exporting KML[edit]

  1. Download and install Google Earth
  2. Move the GE working area to the area of your interest. Wait for streaming to be completed.
  3. The main Tool bar required is as follows
  4. Click Tools -> Option and the Google Earth Option box will be displayed. Check the following boxes : Detail Area = Large, Graphic Mode=OpenGL, Show Lat/Long=Degrees, Show Elevation=Meters, kilometers
  5. Click View -> Sidebar to make sure that the side bar is displayed. Under Layer, make sure that the Terrain box is unchecked.
  6. Zoom the area of interest to the desired Eye alt (look at the lower right corner).
    1. Low resolution area is only good for rivers. The eye alt = about 800m.
    2. High resolution area is ok for digitising plygons (lakes, rivers, buidings) and polylines (roads, trails or other linear stuff) : The eye alt should be between 200 - 300m. Just make sure that the GE image don't pixelate

Digitising A Polyline[edit]

  1. Click Add Path Tool. Change the colour of the line by clicking on the colour palete and select a relevant colours and change the line width of the track to a relevant width. (I like yellow with width of 2). Give a relevant name to the track (Do not close the Box.)
  2. Start digitising by clicking the mouse along the feature that you want to digitise
  3. Click OK when finished with the digitising
  4. Click File --> Places --> Save Places As
  5. Select Save Type as = .kml

Digitising A Polygon[edit]

  1. Click Add Polygon Tool, change the colour and line width of the track and give a relevant name to the track (Do not close the Box.). Then select Outlined.

  1. Start digitising by clicking the mouse along the feature that you want to digitise
  2. Click OK when finished with the digitising
  3. Click File --> Places --> Save Places As
  4. Select Save Type as = .kml

Using KML file as source for Mapping[edit]

Your KML file is similar to the tracks created with a GPS receiver. You can upload it onto Malsingmaps feedback forum as a contribution in return for the latest unlock codes or you can continue to use it as sourse data for your own map.

  1. Use Fishtrack KML to GPX convertor to convert your path traces to GPX format. Please note that Fishtrack cannot convert polygons created in GEPlus to gpx, so use unclosed paths to define areas, instead of polygons.
  2. Use GPSBabel to convert your trace from GPX to Ozi Route format. Please make sure you have the .rte extension in your file name.
  3. Use OziOffset to calibrate your trace to match actual tracks. Note: 0.00001 degree = approx 1.2 metres.
  4. Use GPSMapedit to draw your map based on the trace.

Other Methods[edit]

There are 4 other methods which our members learned from experience. The difficulty level of using these methods depends on your own competency in operating your PC :)

Method 3 advocate tracing and saving the tracks as KML file and then using GPSBabel to convert into GDB or GPX format which can then be treated as typical tracklogs.

Method 1, 2 and 4 advocate saving a snapshot of the satelite image, calibrate it with coordinates and then tracing with it. These 3 methods differs in method of image capture and determining calibration coordinates.

Method 1 - Tracing in GPSMapEdit from Saved GE Images[edit]

GE Setting[edit]

In GE, go to Tools->Options. Then select the View tab. Ensure the following settings:

Detail Area: Large (1024x1024)

Rendering-Lat/Lon: Degrees

Rendering-Elevation: Meters, Kilometers

Prepare Image[edit]

  • Search for the city of your choice (Kuala Lumpur, Singapore, etc).
  • Make sure the image is aligned to true north by clicking on the N button on the navigation panel.

Things to do[edit]

  • Disable all layers, especially the Terrain Layer. Press Ctrl-L to toggle the grid lines to ensure they are straight as shown below:

  • GE works by incremental details. The more you zoom, the clearer the picture. Zoom down to 500m and pan around the target area to download the picture data.
  • When it is done, zoom up again for the appropriate Eye Altitude to same the image. For KL, 1 - 1.2 km is recommended.

Screen Capture[edit]

  • Press F11 to enter full-screen mode.
  • Move your mouse pointer to the left of the screen until the Longitude reading stops. If your image is aligned to true north, the Longitude reading should remain constant (or changes by only 0.000001) when you move the pointer up and down along the left side. (1)
  • Side 2 - Move your mouse pointer to the right of the screen until the Longitude reading stops. Again, the Longitude reading should remain constant (or changes by only 0.000001) when you move the pointer up and down along the right side. (2)
  • Side 3 - Move your mouse pointer to the bottom of the screen until the Latitude reading stops. The Latitude reading should remain constant (or changes by only 0.000001) when you move the pointer left and right along the bottom. (3)
  • Side 4 - The top side is tricky due to the menu bar. If the pointer is on the menu bar, the pointer is displayed as an arrow. If it is on the image, it becomes a hand. The trick is to move slowly from the menu bar to the image until the pointer becomes a hand. Then take down the Latitude reading. This task need practice. When you start, take a few readings to make sure you have the correct top coordinate. (4)

Calibrating the Image[edit]

  • Open up OziExplorer. If you don't have a registered copy, you need to convert the image to BMP format before proceeding (alternatively, you can use OziExplorerTrial version that quits after one hour but can read JPG directly).
  • Go to File->Load and Calibrate Map Image
  • You need to calibrate 2 or 4 points:
    • Point 1 = (0,0) = Top Latitude (4), Left Longitude (1)
    • Point 2 = (1023,742) = Bottom Latitude (3), Right Longitude (2)
  • You can make 2 more calibrations:
    • Point 3 = (1023,0) = Top Latitude (4), Right Longitude (2)
    • Point 4 = (0, 742) = Bottom Latitude (3), Left Longitude (1)
  • Make sure your coordinates have the correct H (N for Latitude, E for Longitude)
  • Save the map.

Another Calibration Method[edit]

Another method to calibrate an image quickly and accurately is to

  • first save the Google Earth image with the Grids activated
  • A minimum of 2 points is needed
    • Look for a GRID LINE intersection on the top left quadrant of the image.
    • calibrate that point according to latitude and longitude readings you see on the lines
    • Then do the same for the second point which should be Diagonally across. For example, if the first grid point was in the top left quadrant, the second grid point should be at the bottom right quadrant.
  • Save the map

Adjustment in Mapedit[edit]

  • For KL, the calibrated image is still about 0.00015 off to the west so we need to move the image a bit.
  • Add the image and save your map.
  • Edit your mp file using a text editor and move to the bottom.
  • You should see this:
  • Make the following changes:
  • Save and Open the map again using Mapedit. Your image is now ready for mapping.

Method 2 - Tracing in GPSMapEdit via View GE from MapSource[edit]

1. Mark and note down two waypoints (any waypoint will do, preferably near the top left and bottom right) in Mapsource, then choose View in Google Earth.


2. Once Google Earth has automagically zoomed in to your two waypoints, hit Print Screen on your keyboard and paste the image in Paint. Save as BMP.


3. Calibrate in OziExplorer


4. And finally draw to your hearts content in good ol' GPSmapedit.


See "Tracing in GPSMapEdit via View GE from MapSource".

Method 3 - Tracing in GE (not GE Plus) via browsers javascript[edit]

  1. Go to this link. There are instructions on how to add a particular javascript bookmark to your browser.
  2. Once you've done that, go to this link
  3. Click on "Satellite" button on the top right of the map window.
  4. Zoom/Pan to the area which you wish to trace.
  5. Click "Start recording" and start tracing! You can pan by dragging. Double clicking automatically adds a point to the path.
  6. Now click on the "GMapToGPX" bookmark which you have created.
  7. The raw code for GPX files will be displayed on the screen.
  8. Copy and paste this text to Notepad and save it as a *.gpx file.

But what about multiple roads? One can only do one road at a time with the method above. The solution which is a bit trickier

  1. Go back to your browser with the raw GPX code. Click the blue "Close" button on the top. You will be returned to the map.
  2. Click on "Clear path and start over"
  3. Trace a new path.
  4. Click on the "GMapToGPX" bookmark again.
  5. Copy the data between the <rte> </rte>
  6. Paste it into the existing GPX file after the previous </rte> but before the </gpx>
  7. Repeat the process for all paths you wish to trace
  8. Save as *.gpx file

Continue the process using Using KML file as source for Mapping

Be careful with the raw data of the gpx files as incorrect placement or missing tags such as <rte>, <gpx>, </gpx>, etc will result in an error when you try to open the file. Sorry no images as of yet, but I'll be glad to help anyone who asks.

See "Mapping with Google Earth (not GE Plus)".

Method 4 - Tracing in GE (not GE Plus) via program GoogleMV[edit]

You can use program GoogleMV (english language is avaliable). Please download program from . This program dump google map (Sat/Map/Hybrid) in OziExplorer format. After dump you can use google map in GPSMapedit to tracing own map.

Checking your Map[edit]

There is a useful free utility that enables you to convert your map in its .mp format to a .kml file. Then you can open this directly in GE, and use all the GE tools to check your map. It does require you to load a free Perl interpreter. See MP2KML