Offline Field Survival Map
Save an area before the trip; in the field, with no network, see where you are and the nearest water, village, road, clinic and high ground, with the slopes and stream beds to avoid.
The phone finds its position from the satellites without any network; what it lacks in the field is the map and what is on it. Before the trip, with internet, an area of 10 to 50 km is saved on the device: the rivers, streams, lakes, springs and wells, towns, villages and houses, roads, tracks and footpaths, clinics, police, shelters and peaks of the open map database; a 30 m elevation model with relief shading, slope and the drainage network computed from it; and the rain, temperature and soil wetness of the week around the saving. In the field the tool opens from the device with no internet, places the GPS position on that map and lists the nearest spring or well, river or lake, stream, village, town, road for a car, paved road, clinic, police, shelter and peak, each with an arrow that turns with the compass, the straight distance, the bearing, the walking time from the slope and the height to climb. It reads the ground where you stand — the slope, the steep slopes above you, whether you are in a stream bed and how large its catchment is, the recent rain and soil wetness, the border strip — gives the sunrise and sunset computed from astronomy, records your path and points back to its start, and sends the position by SMS, which works on the phone network without internet.
Overview
Most accidents in the field come from the same few things: no water, getting lost, darkness, a flash flood in a valley, a fall on steep or wet ground. The tool keeps the answer to each of them on the phone, where it is needed when the network is gone.
Several areas can be kept at once and opened at any time; the last one opens by itself, even with no internet. Ready areas for the north of Iraq are saved with one tap.
What you enter · What you get
What you enter
- A ready area, your location or a point tapped on the map, and the size of the square (10–50 km)
- In the field: the GPS position and, if you wish, the compass
What you get
- Offline map with relief, water, villages, roads and services
- Nearest of 11 needs with arrow, distance, bearing, walking time and climb
- Steep slopes and stream beds; the ground where you stand
- Rain and soil wetness at saving; sunrise and sunset
- Your path and the way back; position by SMS
How it works in the field
Method and equations
Walking time
Tobler’s hiking function gives the speed on each 50 m of the straight line from the slope of the elevation model; the total is raised by a quarter for the winding of real paths.
Slope and relief
Slope from Horn’s 3×3 gradient of the 30 m Copernicus elevation model; relief shading with the sun at 315° and 45°. Slopes of 25–35° are marked as steep and above 35° as very steep.
Stream beds
Each cell passes its area to its steepest lower neighbour (D8) in order of height; cells that drain more than 0.5 km² are marked as stream beds that can carry a flash flood, and those above 10 km² as major ones.
Sun times
Sunrise and sunset from the solar position equations (NOAA), with the standard −0.833° for refraction and the solar disc, so they need no network.
Accuracy and error limits
- The position is as good as the phone’s satellite receiver: 3–10 m in the open, 20–50 m in deep valleys and beside cliffs, and slower to find the first time without network. Distances are straight lines; the walking time is a guide (±30 %) and does not see cliffs, rivers without a bridge or closed land. The elevation model has 30 m cells, so small cliffs and narrow gullies are smoothed. The compass arrow is within about 5–10° after calibration.
- What is not covered: places missing from the open map database (springs, wells and new tracks in remote mountains are often missing, and some streams marked are dry), the weather after the saving, live floods and closures, mines and unexploded ordnance (only a general warning within 15 km of a border), and animal danger, which no map records.
- To reduce the error: save the area the evening before so the rain and forecast are fresh; open the saved area once at home in flight mode to be sure it works; let the GPS settle a minute in the open; ask local people about water and the way; and always carry a paper map, a charged power bank and enough water.
Who uses it
Standards and references
- Tobler, W. (1993). Three presentations on geographical analysis and modeling: non-isotropic geographic modeling. NCGIA Technical Report 93-1.
- Horn, B.K.P. (1981). Hill shading and the reflectance map. Proceedings of the IEEE, 69(1), 14–47.
- O’Callaghan, J.F. & Mark, D.M. (1984). The extraction of drainage networks from digital elevation data. Computer Vision, Graphics, and Image Processing, 28(3), 323–344.
- European Space Agency (2021). Copernicus Global Digital Elevation Model (GLO-30).
- Meeus, J. (1998). Astronomical Algorithms, 2nd ed. Willmann-Bell.
- Geneva International Centre for Humanitarian Demining (GICHD). Landmine and explosive ordnance risk education guidelines.
At a glance
- Category
- Field geology
- Runs on
- Phone, tablet and desktop browser
- Export
- Map sheet as PNG, JPG and PDF; nearest places and your path as GPX and CSV; position by SMS
- Access
- Free
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