Using GPS in off-grid situations.

by tonytran2015 (Melbourne, Australia).

#find North, #tine #GPS, #GPS altitude, #GPS coordinates, #GPS navigation, #off grid, #topographic map, #longitude, #latitude,
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 such as when going hiking.

This posting shows how to do it with minimum battery use.

1. Requirements:

a. A phone with GPS hardware preferably with back up battery or re-charger.

b. A procedure to minimize battery usage.

c. A map with longitude and latitude coordinates printed on paper or stored in the phone (Section 8 will show how to draw those lines on any paper map).

d. An optional software to display your position on the map.

Map with Coordinates

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.

2. Selecting a phone with good off-grid GPS capability.

I would choose one that can quickly obtain an accurate GPS value from cold start. Different phoneMarkingsand GPS apps have different sensitivities, algorithms and accuracy.

Compass app and barometer app are highly desirable additions on such a phone.

Compass app using magnetic sensors gives you the direction without having to move at speed to obtain the deduced True North.(GPS apps obtain True North using solely the tiny increments from GPS coordinates). You should know how to calibrate a magnetic compass app by the figure 8 motion.

Barometer app gives your accurate altitude change if you need that value for working out your position using angles and changes in altitude. Barometer app can also warn you of developing severe weather.

 

3. Two different pieces of hardware can give out two different GPS readings.

The following are the readings from two different smart phones placed in one place. The readings have differences of about 4m in distances in both longitude and latitude directions. (Each minute of latitude angle is one nautical mile, and each second of latitude distance is about 3m in length).

Fig 1&2: Readings from the first smart phone.

Fig 3&4: Readings from the second smart phone running the same app at the same place!

The hardware is guaranteed to give you only some accuracy. My two smart phones seem to have horizontal accuracy values of no better than 4m. Their vertical accuracy values are not better than 10m.

4. A single point may correspond to different latitude and altitude values depending on the different base surfaces used for mean sea level.

Remember that the earth is not a perfect sphere. Its rotation and non-uniform interior density creates its non-spherical surface at sea level. Vertical lines do NOT converge at the center of the earth but are only the lines at right angle to the local sea surface. The latitude and longitude of a point as determined by sextants only give the direction of its vertical line, not the intercept between the non-spherical surface and a line from the center of the earth pointing in that direction!

With that idea in mind, it is clear that a single point in space may have different latitude and altitude values depending on different type of non-spherical surface used for mean sea level.
So you should choose apps with a widely accepted base surface (such as WGS84) so that their latitude and altitude values are readily interchangeable.

Figure 5,6: Reading from GPS Test (1.3.2).

Fig 7: Reading from GPS Status (5.3.112) on the same smart phone.  The two apps give matching readings.

5. When communicating your coordinates with others, you may have to mention the name and version number of your GPS app or the type of base surface in use!

For the same reason, the 3D coordinates from your GPS apps may be inconsistent with the published coordinates of international airports or the land marks on your trips.

Any map with GPS capability such as Googlemap should be already compatible with its GPS in use.

6. Using AGPS updates to save battery.

The phone app can only listen to one GPS satellite at a time. There are many of them and only four clearest transmissions are useful (The others are either out of view or do not provide positional precision). To listen to every transmission and select the clearest four takes time. Then starting from the far off position will take a lot of time for the calculation algorithm to arrive at the correct GPS position.

Remember that the more time you have to wait for a GPS fix the more you run down your battery.

AGPS (Assisted GPS) provides the app with the list of the best transmissions to listen to (from your approximate position) and the algorithm will begin from your approximate position. You will save a lot of time and battery. Choose a GPS app with off-line AGPS update as your principal GPS app.

7. Procedures to use GPS apps

a/- Beware of the high battery usage by GPS hardware.

b/- Run your magnetic compass calibration apps if you think that compass directions may be required.

c/- Run your principal GPS app (with off-line AGPS update capability). Once a fix has been obtained update its AGPS immediately; this saves battery on future fixes.

d/- Run any other GPS apps with your desired features (such as inclination, altitude, magnetic heading, light meter, etc…) immediately after the principal one. The hardware can continue from the values previously obtained by your principal GPS app.

e/- TURN OFF the GPS hardware.

f/- Relate your GPS position to the coordinates (longitude and latitude and possibly with altitude)  on the printed map or the map stored in your phone.

g/- Know the horizontal and vertical error of each fix to know your PROBABLE position.

h/- Turn off the phone or put it in stand-by mode to save battery.

8. 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…

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.

2. 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 four such fearures located near the four extreme corners of your map. They will be used as your four land marks.

Ecample:

The 3 landmarks in use 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

3. 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 four land marks.

Figure: Melbourne Tullamarine and Moorabin airports on a grid map.

4. Check that the shape of the quadrilaleral formed by the four land marks is similar in both the grid sheet and the map. You map have to stretch  or shrink the grid vertically and then horizontally to have a fit. The quadrilaterals should be similar if no mistakes have been made. If the shapes are similar you can proceed to the next step.

5a, If the map and grid are both digital, they can be superimposed to produce the final map:

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.

5b. If you are using printed map: Join two distant landmarks on the coordinate sheet and notice where the lines of “second of longititude” and of “second of latitude” intersect it

6. Reproduce that line, with all its intersecting points, on the actual map.

7. From these intersecting points project corresponding vertical lines to make the  “second of longititude” lines and horizontal lines to make “second of latitude” lines.

8. Your map is now graduated with longitude and laritude lines.

9. Using GPS with maps. 

I prefer 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.

If you want to use your GPS with any map, you need to draw on top of it an accurate system of regularly spaced longitude and latitude coordinate lines as instructed in Section 8.

You can download a 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.

10. Integrated GPS and map apps.

GoogleMap is one such app. It automatically supply you with a map and works out your position from its included GPS. It is great when you are connected. Some old versions of Googlemap allow you to store a map of an area of about 30kmX30km for off-grid use. I don’t know if you can store maps for much larger areas.

I would keep a printed map with coordinates for back up when going off-grid since I would not put everything into one smart phone.

 

11. Error from GPS is rare but not impossible.

As with anything, continuity (or consistency) of readings must always be applied to check for any sudden error arising. It has been reported by BBC news that during the decommission of the GPS satellite SVN23, “some GPS positioning would have been thrown off by nearly 4km.”.

 

References

[1]. tonytran2015, Measuring angles and distances for outdoor survival, survivaltricks.wordpress.com,

https://survivaltricks.wordpress.com/2016/06/29/measuring-angles-and-distances-for-outdoor-survival/, posted 29/6/2016.

[2]. tonytran2015, Selecting and using magnetic compasses, survivaltricks.wordpress.com,

https://survivaltricks.wordpress.com/2016/07/09/selecting-and-using-magnetic-compasses/, posted 09/7/2016.

[3]. , BBC News, UK radio disturbance caused by satellite network bug,

http://www.bbc.com/news/technology-35463347, 2 February 2016.

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