by tonytran2015 (Melbourne, Australia).
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Adding Longitude and Latitude Lines to a map.
Locating where you are using the internet is great but there are times when you have no mean to connect to the internet and you have to use GPS for positioning without any assistance via the internet. Such a situation may arise when you have no internet coverage or when going hiking.
If you want to use your GPS off-grid with any map, you need to draw on top of the map an accurate system of regularly spaced longitude and latitude coordinate lines.
This posting shows how to add the lines.
1. Making a map graduated with Longitude and Latitude Lines.
1. Choose a map with your required resolution and range. The resolution differs for different application: For city street maps resolution should be better than 2m, for country town maps, resolurion only need to be better than 5m as houses are widely separated, for touring, exploring maps, resolution can be upto the (1/10) of the visual range, etc…
2. Make sure that the top of the map points to true North. (In some countries, maps are deliberately oriented at angle to true North, probably for security reasons.)
Figure: Openstreetmap for Melbourne with 3 airport landmarks. Map is used under Open License from Open Street Map, the data are owned by Open Street Map Contributors.
3. On that map, find some unmistakable features such as a well known Airport or Helipad, a Light House, a TV Transmission Tower, a Town Hall, Churches, Schools, Cottages in a forrest, sharply defined mountain peaks, trail intersections… with known longitudes and latitudes. Their coordinates are usually given on the Internet or or easily extracted from Google Map or Navigating Apps.
Find two to four such fearures located near the four extreme corners of your map. They will be used as land marks.
The 3 landmarks that can be used here are:
a. Melbourne Tullamarine airport, Elevation AMSL 434 ft / 132 m, Coordinates 37°40′24″S 144°50′36″E
b. Melbourne Moorabin airport, Elevation AMSL 50 ft / 15 m, Coordinates 37°58′33″S 145°06′08″E
c. Melbourne Essendon airport, Elevation AMSL 282 ft / 86 m, Coordinates 37°43′41″S 144°54′07″E
4. On a separate fresh sheet of paper make a coordinate grid of longitude and latitude covering your range.
On this sheet with grid but no map, mark the coordinates of your land marks.
Figure: Melbourne Tullamarine and Moorabin airports on a grid map.
5. Check that the shape of the figures formed by the land marks are similar in both the grid sheet and the map. You may have to stretch or shrink the grid vertically and then horizontally to have a fit. The figures should be similar if no mistakes have been made. If the shapes are similar you can proceed to the next step.
6a, If the map and grid are both digital, they can be superimposed in the computer to produce the following map with added coordinate lines:
Figure: Openstreetmap for Melbourne with added cooodinates. Map has been modified from original map used under Open License from Open Street Map, the data are owned by Open Street Map Contributors.
The added lines are on round figure coordinates and are 2 minutes of arc apart. The intersection nearest to the NW of Tullamarine airport has coordinate (144°50′E, 37°40′S ).
6b. If you are using printed map: Join two distant landmarks on the coordinate sheet and notice where the lines of “minute of longititude” and of “minute of latitude” intersect it
7. If you are using printed map: Reproduce that line, with all its intersecting points, on the actual paper map.
8. If you are using printed map: From these intersecting points project corresponding vertical lines to make the “minute of longititude” lines and horizontal lines to make “minute of latitude” lines.
9. The map is now graduated with longitude and laritude lines.
10. Its grid can now be used as a base to draw finer grids for detail maps with higher resolutions.
2. Using GPS with maps.
You can download a (preferably topological) map of your area to practice drawing the coordinate lines, the constant altitude lines and learn about the accuracy of the values of longitude, latitude and altitude given by your GPS apps.
It is preferable to use topographic maps with old fashioned land marks (such as churches, tall towers …). Topographic maps give the additional constant ground altitude contours (relative to some mean sea level surface). Constant altitude curves are the faint brown curves on the map illustrated here. The height of each contour is given by a small number. The altitude values of 20m and 10m have been highlighted in this example map by two red circles.
Figure: Opentopomap for the area in my test. Map is used under Open License from Open Street Map, the data are owned by Open Street Map Contributors.
. tonytran2015, Measuring angles and distances for outdoor survival, survivaltricks.wordpress.com,
. tonytran2015, Selecting and using magnetic compasses, survivaltricks.wordpress.com,
. http://www.bbc.com/news/technology-35463347, 2 February 2016., BBC News, UK radio disturbance caused by satellite network bug,
Added after 2018 Nov 26:
The Thors’ son urged people travelling to remote locations without mobile coverage to download a GPS application to their phones ahead of their journey, as well as an offline map for their destination.
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