Mercator-based navigational fix

`[latfix,lonfix] = navfix(lat,long,az)`

[latfix,lonfix] = navfix(lat,long,range,casetype)

[latfix,lonfix] = navfix(lat,long,az_range,casetype)

[latfix,lonfix] = navfix(lat,long,az_range,casetype,drlat,drlon)

`[latfix,lonfix] = navfix(lat,long,az)`

returns
the intersection points of rhumb lines drawn parallel to the observed
bearings, `az`

, of the landmarks located at the points `lat`

and `long`

and
passing through these points. One bearing is required for each landmark.
Each possible pairing of the n landmarks generates one intersection,
so the total number of resulting intersection points is the combinatorial *n
choose 2*. The calculation time therefore grows rapidly
with n.

`[latfix,lonfix] = navfix(lat,long,range,casetype)`

returns
the intersection points of Mercator projection circles with radii
defined by `range`

, centered on the landmarks located
at the points `lat`

and `long`

.
One range value is required for each landmark. Each possible pairing
of the *n* landmarks generates up to two intersections
(circles can intersect twice), so the total number of resulting intersection
points is the combinatorial *2 times (n choose 2)*.
The calculation time therefore grows rapidly with *n*.
In this case, the variable `casetype`

is a vector
of `0`

s the same size as the variable `range`

.

`[latfix,lonfix] = navfix(lat,long,az_range,casetype)`

combines
ranges and bearings. For each element of `casetype`

equal
to 1, the corresponding element of `az_range`

represents
an azimuth to the associated landmark. Where `casetype`

is
a 0, `az_range`

is a range.

`[latfix,lonfix] = navfix(lat,long,az_range,casetype,drlat,drlon)`

returns
for each possible pairing of landmarks only the intersection that
lies closest to the dead reckoning position indicated by `drlat`

and `drlon`

.
When this syntax is used, all included landmarks' bearing lines or
range arcs must intersect. If any possible pairing fails, the warning ```
No
Fix
```

is displayed.

This is a navigational function. It assumes that all latitudes
and longitudes are in degrees and all distances are in nautical miles.
In navigation, piloting is the practice of fixing one's position based
on the observed bearing and ranges *to* fixed landmarks
(points of land, lighthouses, smokestacks, etc.) *from* the
navigator's vessel. In conformance with navigational practice, bearings
are treated as rhumb lines and ranges are treated as the radii of
circles on a Mercator projection.

In practice, at least three azimuths (bearings) and/or ranges
are required for a usable fix. The resulting intersections are unlikely
to coincide exactly. Refer to Navigation in the *Mapping Toolbox User's
Guide* for a more complete description of the use of this
function.

For a fully illustrated example of the application of this function,
refer to the Navigation section
in the *Mapping Toolbox User's Guide*.

Imagine you have two landmarks, at (15ºN,30.4ºW) and (14.8ºN,30.1ºW). You have a visual bearing to the first of 280º and to the second of 160º. Additionally, you have a range to the second of 12 nm. Find the intersection points:

[latfix,lonfix] = navfix([15 14.8 14.8],[-30.4 -30.1 -30.1],... [280 160 12],[1 1 0]) latfix = 14.9591 NaN 14.9680 14.9208 14.9879 NaN lonfix = -30.1599 NaN -30.2121 -29.9352 -30.1708 NaN

Here is an illustration of the geometry:

Traditional plotting and the `navfix`

function
are limited to relatively short distances. Visual bearings are in
fact great circle azimuths, not rhumb lines, and range arcs are actually
arcs of small circles, not of the planar circles plotted on the chart.
However, the mechanical ease of the process and the practical limits
of visual bearing ranges and navigational radar ranges (~30 nm) make
this limitation moot in practice. The error contributed because of
these assumptions is minuscule at that scale.

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