Adjusting the Map Scale in OCAD 8


What's the Problem?

The original map was created from a scanned image or picture of unknown scale/resolution - it's not uncommon just to key in a random number at the time to bypass this calibration step...the logic being that one can easily fix it later by adjusting the print scale. The issue here is that OCAD thinks that the internal map scale is as recorded and that you have to experiment to work out what random value you have to put into the print dialogue box to get it to print at say 1:10,000.

How does the Process Described here Help?

This process tells you how to work out what the internal scale actually is. If you plug this value into your map, and then select your required print scale...your map will print at that scale. ie if you set the print scale in the print dialogue box to 1:10,000, then your map will be printed at 1:10,000. If it doesn't fit well on the page, just adjust the scale to 1:12,500 (or whatever) and it will resize the print image (ie not the map) to print at the new scale.

How do I know if there's a Problem?

Use the MEASURE tool in the toolbar...

If you select one or more line objects within the map and then press the toolbar button that looks like a ruler, the MEASURE RESULT dialog box appears. This dialogue box tells you the length of the line (or lines) selected.

The Real World value should match the equivalent measurement taken from the Melways (note that the standard Melways maps are 1:20,000 - ie to calculate real world distance, measure the length in the Melways in mm and then multiply by 20,000. This gives the Real World distance in mm - to convert this to metres, divide by 1000 (Tip - measure the Melways in mm and multiply by 20 to get to metres).

Background - Scale Functions Available in OCAD

This section describes the functions that you will need to use to fix your map.

Options > Scales

This command sets the internal map scale - this is just a number inside the file. It does not change the size of the map or the symbols within it.

Map Scale
Enter here the scale of the final map. You can choose one of the predefined scales or enter any other scale on the keyboard. For a scale of 1:60000, enter 60000.
Note: Changing the scale of an existing map only changes a number in the map file. If you want to enlarge/reduce the map to a new scale, choose Change scale from the Extras menu.

Paper coordinates: Choose this option to display the grid and the mouse position in paper coordinates. Enter the desired grid distance. Press the grid button to display the grid on the screen.
Real world coordinates: Choose this option to display the grid and the mouse position in real world coordinates. Press the grid button to mark the grid on the screen.
Real world coordinates are measured in meters. Normally the coordinates of the national land survey are used here. For the horizontal and vertical offset enter the coordinates corresponding to the center of the OCAD coordinate system (0, 0) {e.g. You can use this to provide AMG coordinates for your map}.

Extras > Change Scale

This command changes the display scale of the map. The map is enlarged/reduced according to the new scale.

Current scale
This line shows the current scale of the map. Choose Scales from the Options menu to set the current scale. Note: Changing the scale in the Options menu does not enlarge or reduce the map. It only changes a number in the map file.

New scale
Enter here the desired new scale of the map. You may choose one of the predefined scales or enter the scale on the keyboard.

Enlarge/reduce symbols
Check this box to enlarge/reduce the symbols with the same factor as the map. When the box is checked, the map is enlarged/reduced like a photographic enlargement. When this box is not checked, the map is enlarged/reduced, but the symbols are kept in the same dimension.

OK We're Ready - Let's Fix your Map

To illustrate the explanation below, I'll use a sample map - where one has to set the Print Scale to 22,000 to get the map to print at 1:10,000

  1. Select an obvious reference line on your map - measure this distance in the Melways.
            - In my sample map here, the distance was 1.09 Km
  2. Select a line/road or just draw a line in your map that matches the reference line selected above.
            - Use the MEASURE tool to see how long OCAD thinks the line is
            - In my sample map, the ruler says that the line is 2.088 Km long
  3. Select Options > Scales to adjust the internal scale
            - Calculate the internal map scale - as follows
            - New Map Scale = Current Map Scale * Real World Distance / Distance provided by MEASURE Tool
            - In my sample map, I set the Map Scale to 15000*1.09/2.088  =  7830
  4. Under Options > Scales, change the Map Scale to your calculated value. Then press OK - and verify that all is well by re-measuring the reference line (step 1).
  5. Next time you print - just set the scale to 10,000 - it should print at that scale.

Other Notes/Tips

  1. Check the Scale Bar - I just measured the one in the sample map here. While the text says the scale is 1:11,000 and the bar shows a series of steps labeled from 0 to 1000m, the scale bar is actually a little more than 1100m in length (To fix this, I did a marquee select on the scale bar and numeric values along its length - and then dragged the selected symbol to the correct length.
  2. Check that the text description of the Scale is still correct.


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