AP_Logger: add a README.md to help locating units/mults/fieldtypes

libraries/AP_Logger/LogStructure.h

@@ -1,5 +1,7 @@
 #pragma once
 
+// if you add any new types, units or multipliers, please update README.md
+
 /*
 Format characters in the format string for binary log messages
   a   : int16_t[32]

libraries/AP_Logger/README.md

@@ -0,0 +1,102 @@
+# Logger Notes
+
+## Format Types
+
+The format type specifies the amount of storage required for the entry
+and how the content should be interpreted.
+
+| Char | C Type |
+|------|--------|
+|a   | int16_t[32]|
+|b   | int8_t|
+|B   | uint8_t|
+|h   | int16_t|
+|H   | uint16_t|
+|i   | int32_t|
+|I   | uint32_t|
+|f   | float|
+|d   | double|
+|n   | char[4]|
+|N   | char[16]|
+|Z   | char[64]|
+|L   | int32_t latitude/longitude (so -35.1332423 becomes -351332423)|
+|M   | uint8_t flight mode|
+|q   | int64_t|
+|Q   | uint64_t|
+
+Legacy field types - do not use.  These have been replaced by using  the base C type and an appropriate multiplier column entry.
+
+| Char | CType+Mult   |
+|------|--------------|
+|  c   | int16_t * 100|
+|  C   | uint16_t * 100|
+|  e   | int32_t * 100|
+|  E   | uint32_t * 100|
+
+## Units
+
+All units here should be base units
+This does mean battery capacity is here as "amp*second"
+Please keep the names consistent with Tools/autotest/param_metadata/param.py:33
+
+| Char | Unit Abbrev. | Description |
+|-----|---|---|
+| '-' | "" | no units e.g. Pi | or a string|
+| '?' | "UNKNOWN" | Units which haven't been worked out yet....|
+| 'A' | "A" | Ampere|
+| 'd' | "deg" | of the angular variety | -180 to 180|
+| 'b' | "B" | bytes|
+| 'k' | "deg/s" | degrees per second. Degrees are NOT SI | but is some situations more user-friendly than radians|
+| 'D' | "deglatitude" | degrees of latitude|
+| 'e' | "deg/s/s" | degrees per second per second. Degrees are NOT SI | but is some situations more user-friendly than radians|
+| 'E' | "rad/s" | radians per second|
+| 'G' | "Gauss" | Gauss is not an SI unit | but 1 tesla = 10000 gauss so a simple replacement is not possible here|
+| 'h' | "degheading" | 0.? to 359.?|
+| 'i' | "A.s" | Ampere second|
+| 'J' | "W.s" | Joule (Watt second)|
+| 'l' | "l" | litres|
+| 'L' | "rad/s/s" | radians per second per second|
+| 'm' | "m" | metres|
+| 'n' | "m/s" | metres per second|
+| 'N' | "N" | Newton|
+| 'o' | "m/s/s" | metres per second per second|
+| 'O' | "degC" | degrees Celsius. Not SI | but Kelvin is too cumbersome for most users|
+| '%' | "%" | percent|
+| 'S' | "satellites" | number of satellites|
+| 's' | "s" | seconds|
+| 'q' | "rpm" | rounds per minute. Not SI | but sometimes more intuitive than Hertz|
+| 'r' | "rad" | radians|
+| 'U' | "deglongitude" | degrees of longitude|
+| 'u' | "ppm" | pulses per minute|
+| 'v' | "V" | Volt|
+| 'P' | "Pa" | Pascal|
+| 'w' | "Ohm" | Ohm|
+| 'Y' | "us" | pulse width modulation in microseconds|
+| 'z' | "Hz" | Hertz|
+| '#' | "instance" | (e.g.)Sensor instance number|
+
+## Multipliers
+
+This multiplier information applies to the raw value present in the
+log.  Any adjustment implied by the format field (e.g. the "centi"
+in "centidegrees" is *IGNORED* for the purposes of scaling.
+Essentially "format" simply tells you the C-type, and format-type h
+(int16_t) is equivalent to format-type c (int16_t*100)
+tl;dr a GCS shouldn't/mustn't infer any scaling from the unit name
+
+| Char | Multiplier |
+|------|------------|
+| '-' | 0 |       // no multiplier e.g. a string|
+| '?' | 1 |       // multipliers which haven't been worked out yet....|
+| '2' | 1e2 ||
+| '1' | 1e1 ||
+| '0' | 1e0 ||
+| 'A' | 1e-1 ||
+| 'B' | 1e-2 ||
+| 'C' | 1e-3 ||
+| 'D' | 1e-4 ||
+| 'E' | 1e-5 ||
+| 'F' | 1e-6 ||
+| 'G' | 1e-7 ||
+| '!' | 3.6 | // (ampere*second => milliampere*hour) and (km/h => m/s)|
+| '/' | 3600 | // (ampere*second => ampere*hour)|