Starpath Chart Tutorials

To measure a short distance using the Latitude scale...
(1) Set divider tips to span the distance
(2) Move the dividers to the latitude scale on the left or right side of the chart, roughly due east or west of the segment you are measuring.
(3) Set one divider tip on some latitude of a whole number of minutes, such at 00' or 05' or 40'
(4) Let the other divider tip fall onto the latitude scale wherever it might. The second tip can be at a higher or lower latitude that the first since we care only about the distance between them.
(5) Figure the distance using the rule that each minute of latitude equals one nautical mile. Always double check the latitude scale to be sure you understand what the tick mark or graduation marks mean. Sometimes the
latitude scale is marked in tenths of minutes, other times in various units of arc seconds, such as a mark every 5".

General Notes
The latitude scale can be used for a miles scale on all charts, even if they happen to have a specific miles scale printed on them. Remember to go more or less straight left or right to reach the latitude scale — and that this must be measured from the latitude scale (sides of the chart), not the longitude scale (bottom and top of the chart).

That is, do not set dividers to some distance near the top of the chart and then put them on the latitude scale near the bottom of the chart. On a small scale chart which covers a large area, if you make a shift like that you can introduce significant errors in the distance measurement.

A note on the reason for this: Nautical charts are made in what is called the Mercator projection. This type of projection is very convenient for navigation since North is always the same direction on a chart — usually toward the top of the page — and all bodies of land and water have the proper shape. This latter point is important since it lets us determine what the compass bearing is from one point to another from the chart.

This seemingly obvious task is not so simple as it might seem. Near the North Pole for example, or at any very high latitude, this type of chart (Mercator) is not possible and consequently it is difficult to determine bearings or directions from a chart in these areas. Remember if you are standing at the North Pole, every direction is South! — a good hint that we have a charting problem in this region.

In any event, away from the polar regions, Mercator charts serve us well for navigation, but there is a price to pay for the bearing convenience. Although the shapes of lands are right, their relative sizes are not. Greenland is not larger than the US as it appears on a Mercator projection, it is in fact less than one quarter the size of the US. So it is with all land masses on Mercator charts. They are bigger at higher latitudes. Put another way, the number of inches per degree of latitude or per nautical mile is higher at higher latitudes.

It is always true that 1 minute of latitude = 1 nautical mile, but the physical size of this unit will vary with latitude. That is why when using small scale charts (those that cover a large area) we must use the nearest latitude scale to measure distances.

Video Notes
This is the same measurement explained in the previous example (Short line with miles scale), but this time we use the latitude scale to read the miles. If you have not done so, please read through the previous example first.
After setting the separation, we move the dividers straight left (or right) to the nearest latitude scale along the side of the chart. Here we set the top tip to the latitude of 37° 20' N and let the bottom one fall where it will. We chose this one simply because it was the nearest whole value due west of our measurement. This particular latitude scale is marked with broad bands every 1' of latitude. Although the numbers are not shown, the bottom tip is at latitude 37° 17.4' N.

What we care about here, though, is only the separation, which we can count down from the 20' mark to see that it is 2.6' = 2.6 nautical miles.

In this very common operation of reading a latitude, or in this case a latitude interval, it is very important that we first check to see what the tick marks mean along the scale. Don't guess this, always double check it. Different charts use different conventions on what is bold and how many minutes or seconds there are per tick.

We usually check this carefully once, and then actually write it on the chart. That is, had we used this chart before, there would be a "19' " label and an "18' " label hand drawn in along the border. On this chart, the next printed label is located out of sight here at 37° 15'.

Starpath Inland and Coastal Navigation Course Short distances with the latitude scale
Measure Distances • Short distances with the miles scale • Short distances with the latitude scale • Long distances in steps Measure Directions • Magnetic direction using parallel rulers • True direction using Weems plotter • True direction using a string plotter Plotting positions and courses • Using Lat - Lon scale • Plotting COG from a plotted position • Using the Starpath card plotter method
To measure a short distance using the Latitude scale... (1) Set divider tips to span the distance (2) Move the dividers to the latitude scale on the left or right side of the chart, roughly due east or west of the segment you are measuring. (3) Set one divider tip on some latitude of a whole number of minutes, such at 00' or 05' or 40' (4) Let the other divider tip fall onto the latitude scale wherever it might. The second tip can be at a higher or lower latitude that the first since we care only about the distance between them. (5) Figure the distance using the rule that each minute of latitude equals one nautical mile. Always double check the latitude scale to be sure you understand what the tick mark or graduation marks mean. Sometimes the latitude scale is marked in tenths of minutes, other times in various units of arc seconds, such as a mark every 5". General Notes The latitude scale can be used for a miles scale on all charts, even if they happen to have a specific miles scale printed on them. Remember to go more or less straight left or right to reach the latitude scale — and that this must be measured from the latitude scale (sides of the chart), not the longitude scale (bottom and top of the chart). That is, do not set dividers to some distance near the top of the chart and then put them on the latitude scale near the bottom of the chart. On a small scale chart which covers a large area, if you make a shift like that you can introduce significant errors in the distance measurement. A note on the reason for this: Nautical charts are made in what is called the Mercator projection. This type of projection is very convenient for navigation since North is always the same direction on a chart — usually toward the top of the page — and all bodies of land and water have the proper shape. This latter point is important since it lets us determine what the compass bearing is from one point to another from the chart. This seemingly obvious task is not so simple as it might seem. Near the North Pole for example, or at any very high latitude, this type of chart (Mercator) is not possible and consequently it is difficult to determine bearings or directions from a chart in these areas. Remember if you are standing at the North Pole, every direction is South! — a good hint that we have a charting problem in this region. In any event, away from the polar regions, Mercator charts serve us well for navigation, but there is a price to pay for the bearing convenience. Although the shapes of lands are right, their relative sizes are not. Greenland is not larger than the US as it appears on a Mercator projection, it is in fact less than one quarter the size of the US. So it is with all land masses on Mercator charts. They are bigger at higher latitudes. Put another way, the number of inches per degree of latitude or per nautical mile is higher at higher latitudes. It is always true that 1 minute of latitude = 1 nautical mile, but the physical size of this unit will vary with latitude. That is why when using small scale charts (those that cover a large area) we must use the nearest latitude scale to measure distances. Video Notes This is the same measurement explained in the previous example (Short line with miles scale), but this time we use the latitude scale to read the miles. If you have not done so, please read through the previous example first. After setting the separation, we move the dividers straight left (or right) to the nearest latitude scale along the side of the chart. Here we set the top tip to the latitude of 37° 20' N and let the bottom one fall where it will. We chose this one simply because it was the nearest whole value due west of our measurement. This particular latitude scale is marked with broad bands every 1' of latitude. Although the numbers are not shown, the bottom tip is at latitude 37° 17.4' N. What we care about here, though, is only the separation, which we can count down from the 20' mark to see that it is 2.6' = 2.6 nautical miles. In this very common operation of reading a latitude, or in this case a latitude interval, it is very important that we first check to see what the tick marks mean along the scale. Don't guess this, always double check it. Different charts use different conventions on what is bold and how many minutes or seconds there are per tick. We usually check this carefully once, and then actually write it on the chart. That is, had we used this chart before, there would be a "19' " label and an "18' " label hand drawn in along the border. On this chart, the next printed label is located out of sight here at 37° 15'.
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