Georeferencing means electronically adding coordinates to an electronic copy of a weather map so you can read lat and lon from a cursor, measure range and bearing on the maps, or have your GPS position show up directly on the maps. There are a few commercial products that do this provided you use their own charts and software, but there is a simple way to do this using the chart viewer mode of a very nice program and a few simple instructions.

A very convenient way to do this is to use the powerful Memory-Map Navigator program that we use in our electronic navigation training. The Memory-Map company promotes the use of this program by offering it as a free chart viewer, because they know that once you learn how simple and convenient it is to use, you may want to purchase it for your main navigation underway. Without purchase it will not accept a live GPS and you cannot load it into a PDA—if you care to look into what we call "ultra mobile navigation." You might argue there are better programs for PC navigation in some applications, but when it comes to PDA navigation, this program is the top of the line for that application. But i think you will also find it a very attractive PC echart system once you learn how simple, yet versatile it is. This is just one example. Below we give samples of georeferenced Pilot Charts, Current Charts, and even screen captures from Google Earth. It will also show AIS data right on the charts as well (in the purchased professional version). We used georeferenced Gulf Stream charts in Memory Map thoughout the 2007 Trans Atlantic Rowing Race to support the Seattle team that won the race by more than a week, setting a new world record for ocean rowing.

Here is a view of a surface analysis map of the Atlantic within MM-Nav.

To load and prepare a map follow these steps:

(1) Get your map. For Atlantic, Pacific and Gulf Regions, best place is often the Ocean Prediction Center (Google "NWS OPC"). It will download as a gif, but open it in something like Paint Shop Pro and then save it as a tif (or from Start button, select Run, type mspaint, and use that program, but it helps to have something like paint Shop Pro or even Adobe Photoshop for the graphics operations, but mspaint will make this simple conversion.) It is best to make a separate folder for these maps that you intend to georeference, and download and convert to tif in that folder, such as c:\myMMmaps. When you are done with this step, you will have a folder called c:\myMMmaps and in this will be myfirstmap.gif and myfirstmap.tif.

(2) Open MM-Nav, then open Map link, then Refresh Map List, navigate to c:\mymaps, select Load all maps. Select and load the tif map.

(3) Then under the Map menu, select calibration and add the georeferencing. My experience is it is best to do the four corners of the map, then a the mid-longitude line of the map insert one calibration point every 10° of lat. Read the instructions. You can just click a point and then type in (say) 40 N, 120 W and save. It might not come in right at first, but then the format is there and you can correct the values. Once done, check a few points my scanning around with the cursor.

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Once you have a map georeferenced you can read off the isobar spacing by setting a two point route across them. Likewise you can now predict positions accurately and easily. if you have georeferenced a 96-hr map for example you can plot your present position and then use the route tool to plot your position at the valid time of the map. Below is a sample of a Navy Gulf Stream analysis. These are available every 3 days.

You can download the above files already georeferenced to practice with in your copy of Memory Map Navigator:

Surface analysis, Gulf Stream Analysis , and Oct Pilot Chart section

In the pilot charts, the green arrows are the current drift in knots, the wind roses show speeds in Beaufort Force numbers, each half of a tail feather is one number... much of the trades you see here are force 4. The length of the arrows are the probabilities. You would use a chart like this georeferenced to make a quick estimate of the winds to expect and the current set along a route. The Pilot Charts are available online as pdfs. You can extract the map files from the pdfs (if you have a copy of Acrobat) or do a screen capture for sharper images.

We will come back to the Google Earth imaging later. Key points are be sure the image is exactly north up on the screen before capturing, and add a pin to each of the corners and note the lat-lon of each as you plant it before capturing. Then just carry out as above. The book Modern Marine Weather includes a section on use of Google Earth for several weather map overlay applications.