HOME    OZIEXPLORER  GPS TRACK   OCAD  LINKS & MAPPERS

 

Geo-referencing an OCAD map

Updated Dec 23rd 2011

This is always easier to achieve as a first step before drawing a single benzier curve on your map!

A) Calculating the correct Rotation of a map. (Thanks to Adrian & Robin Uppill for advice on writing this section with inspiration from Ken Dowling's article ( from which I have borrowed a diagram..thanks Ken) "Magnetic Variation a No No" 

  • First check the magnetic declination for the area you are mapping. This is the angle between True North and Magnetic North. This can be done by using the online calculator on the Geoscience Australia website. You should use latitude and longitude figures for this. This is most accurate within the Australian continent but calculations for other parts of the world can be done here using a global magnetic field model. For example it is currently 8.15 degrees for Adelaide South Australia. Ensure you enter the current date as this varies over time.
  • Next work out the Grid Convergence. This a small angle between the north lines of the grid and lines of longitude (True North).
Below is a link to the Redferns Formula which is also found on the Geoscience Australia website. The UTM coordinates are used for this.
http://www.ga.gov.au/geodesy/datums/redfearn_grid_to_geo.jsp
In the above example using Adelaide South Australia the Grid Convergence is -1 deg 21 mins 24 secs (-1.36 degrees)
  • Finally calculate the Grid Magnetic Angle using the following formula:
Amount of Rotation(GMA) = Magnetic Declination + Grid Convergence
In the example above GMA = 8.15 + (-1.36) = 6.79 which you would round to 6.8 degrees

Other calculators you may find useful in above process:
a)to convert Grid coordinates to latitude and longitude
(geographic) 
-	http://www.ga.gov.au/geodesy/datums/redfearn_grid_to_geo.jsp
and geographic to Grid at
-	http://www.ga.gov.au/geodesy/datums/redfearn_geo_to_grid.jsp

B) Using photogrammetry 

  • This is always the preferred method with a new map as it should provide the greatest accuracy and compatibility to geo-tiff photography.
  • Open a new OCAD map using the appropriate scale and symbol set.
  • Import ( file > import) filename.dxf  into the map file using the rotation figure (GMA) obtained above and selecting the default New Offset indicated.
  • If you are also using a geo-referenced photograph (tiff file) you should now be able to open this as a background map using the same real world coordinates you have used above.
  • If you now go to Options > Scales [OCAD 9] or Map > Coordinate System [OCAD 10] click on the Real World Coordinates option.
  • If you now turn on the show grid button you will have an excellent start to your base map that will match co-ordinates produced by your GPS.

B) Using geo referenced photography:

  • Check rotation figure (GMA) as above
  • Obtain a recent geo-referenced photograph of the area.
  • Load the existing map
  • Open the photograph  as a background map placing it using the new offset that appears in the dialogue box but entering the magnetic variation calculated above.
  • Select the entire map and align it with the photograph zooming in to clear reference points to get the greatest possible degree of accuracy.
  • You may find that you need to rotate ( Extras OCAD 9 Map OCAD 10) the map to get the rotation figure (GMA) right. Remember to uncheck the Rotate symbols oriented to north option.
  • If the map does not align closely you can use the Transform Map function to adjust the map. If the differences are very significant and cannot be easily adjusted it may be better to consider re-mapping the area with new photogrammetry. This is not an uncommon problem with older maps that were made using non-rectified photography or even earlier 1: 50 000 maps as a base.

C) Using recorded GPS points and tracks:

I find this method very effective. It is based on the assumption that, with your particular GPS you are able to to accuracy within the 3-6m range. I use a Garmin Oregon 300 and find it useful to make sure the GPS accuracy window is always displayed in the compass window. I always check accuracy before recording a waypoint. If this is not possible the results may be less effective. Begin by recording a range of waypoints at distinct points reasonably spread across your map. The steps involved after that are described very well in Geoff Peck's article (April 2010) Geoff Peck's guide to Geo-referencing O Maps. The only suggestion I would make is that you are better off using distinct waypoints rather than tracks. Also when it comes to understanding how transform works in OCAD 10 there is an excellent flash video that walks you through the process.  

D) Using Google Earth:

Note: Be warned the photography available available in Google Earth is not as spatially accurate as commercially geo-referenced photography. Personally I find method C) above preferable if A0 & B) are not available. However the steps involved are:

  1. Open Google Earth
  2. Under Layers in the left margin of Google Earth, tick on “Roads” to show the roads layer. Roads are usually accurate even if satellite images are not too good. You may choose to use either roads or the satellite images in the following.
  3. Under Tools-> Options -> 3D View Tab  Show Lat/Long & click Universal Transverse Mercator
  4. Go to a point in Google Earth which is on the map and is 100% identifiable (preferably a road crossing or similar). Write down the UTM coordinates of this point – including UTM zone.
  5. In OCAD 10 , go to Map -> Scale and Coordinate System . Click  “Real World coordinates”. Under coordinate system, choose the correct UTM grid . For Horizontal offset and Vertical offset choose the numbers you found
  6. In OCAD 10, go to Map -> Transform Map. Click the point you found previously (when the program says click point on map).
  7. In OCAD, move around the map until the coordinates shown at the bottom of OCAD show the  values you recorded
  8. Repeat with at least two more points (Find point on Google Earth, write down position, click for map point etc.)
  9. Press return in OCAD. Now the points are transformed.
  10. You can check how accurate the transformation is by going over the map with the mouse in OCAD, and observing the transformed coordinates at the bottom of the window in OCAD.