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Toporama and Oziexplorer

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I have my digital maps from Spectrum for most of the areas we go to but just recently read here about the free Toporama maps and calibrations and I've been using it to fill in the areas I don't have and it's great. I do have a couple of questions though.

1) In Ozi, what projection should I select in the calibration? I have Lat,Long but wondered if I should be using UTM - does it matter?

2) Most the map images are created the same size (3200 X 1600) which means when you use a UTM grid, you end up with a rectangle. I understand why this is, but has anyone found a method to set the image size to end up with a more correct image? I know this would be different with different Latitudes since Longitude changes. I now resize at 3200 x 2500 which gets close for some maps.

Toporama maps are created with an equal number of pixels in the horizontal axis regardless of the latitude. This was done to facilitate mosaicing adjoining images without having to deal with the convergence of meridians.

Since Calgary is not located on the equator, the arc distance in longitude for an equal value of latitude, will be compressed by a factor equal to the cosine of the latitude.

Therefore, to correct your 3200 x 1600 map for your area (N 51 degrees), you need to scale the X axis by 0.6293. Your scale corrected image should now be 2014 X 1600. Only scale the X axis, not the Y.

quote:
Originally posted by MrGigabyte:

Toporama maps are created with an equal number...

Thanks for the help understanding this MrGigabyte, I knew there had to be some relationship. Not exactly the fastest method to acquire maps, but for the price, you can't complain!

What do you do when a map image (such as 072M16) shows from Lat 51 45 0.0 - 52 0 0.0. Do you use the cos of 51.5? or just stick with 51 0.0? Can't see that it makes a huge difference for geocaching or hiking anyway, just wondered.

Thanks

I would use the centre of the map. In your example, the scale factor at the bottom is the cos(51.75) = 0.6191. At the top, cos(51.00) = .6293. In terms of pixels, that represents 1981 versus 2014 or a difference of 33 pixels.

1 pixel = 1/72 inch.

1 inch = 0.0254m

0.0254 / 72 * 50000 = 17.6m

In other words, each pixel = 17.6m on your 1:50,000 scale maps.

17.6m * 33 pixels = 580m

Therefore the top plan is shown as being 580m wider than it should be, as compared to the bottom. You can reduce this by half by using the scale factor for the centre of the plan.

This is off the top of my head. You may want to check this with a calculator

quote:
Originally posted by MrGigabyte:

I would use the centre of the map...

Works out just as you said. Thanks again for the help understanding. I should have most of the gaps filled in by the end of the week.

Thanks

Francis

Someone wrote a program that they emailed me that will turn the .dat files into .map files. It shouldn't be that hard to call a resizing function and resize the file too.

MrG, your a programmer right?

Rob

Mobile Cache Command

2) Most the map images are created the same size (3200 X 1600) which means when you use a UTM grid, you end up with a rectangle. I understand why this is, but has anyone found a method to set the image size to end up with a more correct image? I know this would be different with different Latitudes since Longitude changes. I now resize at 3200 x 2500 which gets close for some maps.

use your printed maps to find the ratio of the real maps in your latatude it should be 1 : 1.725 and it makes the map 1600x2760

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