About the previous map, but in a separate post...
- As you can easily see, it's a Mercator projection or, to be more specific, a Web Mercator, although the difference is negligible at this scale. The base map is Google Maps, with the Google Maps Layer hidden before exporting to PNG.
- The form factor is 1:2 because the map is cut vertically at about 74°N and 56°S.
- The horizontal cut is not at the OTL 180° meridian but through the Bering Strait, having all of Asia on the right side of the map. This is because in TTL, the meridians are numbered differently.
TTL Longitude
While the parallels are counted as in OTL (from 90°N = the North Pole to 0° = the Equator to 90°S = the South Pole),
the meridians are counted from 0° to 360°, without an East / West designation.
Greater numbers are Eastwards, smaller numbers are Westwards. TTL Meridians are in
Black, OTL Meridians are in
Red.
The
0° (
168°40'W) or the
Date Line Meridian passes through the
Bering Strait (the leftmost line on the map).
The
168°40' (
0°) meridian passes through Greenwich.
The
180° or the
Central Meridian (
11°20'E) passes through the heart of Europe (Scandinavia, Germany, Italy).
The
348°40' (
180°) meridian is our antimeridian.
The
360° (
168°40'W) is identical with the
0° meridian (the rightmost line on the map).
To convert from TTL Longitude to OTL Longitude:
1. Add 11°20'.
2a. If less than 180°, substract from 180° and append W (OTL Western Hemishpere).
2b. If between 180° and 360°, substract 180° and append E (OTL Eastern Hemisphere).
2c. If greater then 360°, substract 360° then substract the result from 180° and append W (OTL Western Hemisphere).
Example 1: The vertical border between Mexico and British North America (
73°50').
1. Add 11°20', result 85°10'.
2a. Substract from 180°, result
94°50'W.
Example 2: The border between Mauretania and Libya (
174°).
1. Add 11°20', result 185°20'.
2b. Substract 180°, result
5°20'E.
Example 3: The border between Libya and Egypt (
194°).
1. Add 11°20', result 205°20'.
2b. Substract 180°, result
25°20'E.
Example 4: The vertical border between Russia and Scandinavian Siberia (
230°40').
1. Add 11°20', result 242°.
2b. Substract 180°, result
62°E.
To convert from OTL Longitude to TTL Longitude:
1a. If Western, substract from 180°.
1b. If Eastern, add 180°.
2. Substract 11°20' (and, if negative, add 360°).
Example 1: New York (
74°W)
1a. Substract from 180°, result 106°.
2. Substract 11°20', result
94°40'.
Example 2: Paris (
2°21'E)
1b. Add 180°, result 182°21'.
2. Substract 11°20', result
171°1'.
Example 3: Jerusalem (
35°13'E).
1b. Add 180°, result 215°13'.
2. Substract 11°20', result
203°53'.
I hope that a little maths now and then isn't excessively boring and does not chase my readers away.
P.S. Just in case someone wonders...
Question: It's too early for advanced chronometres for precise longitude measurements. How were they able to determine the longitude of the Bering Strait with that precision?
Answer: They got lucky. When a Scandinavian ship wintered at the Bering Strait, there happened to be a Moon eclipse and the exact longitude could be obtained with an elementary astronomical calculation when they arrived back home!