METAR & TAF Weather Codes — AviationRef.com

METAR Format Overview

A METAR (Meteorological Aerodrome Report) is an hourly weather observation for an aerodrome. A SPECI is a special (non-routine) observation issued when conditions change significantly. Both follow the same format.

EGLL 121250Z 27015KT 9999 FEW020 15/08 Q1012 NOSIG

EGLL = ICAO station identifier (London Heathrow)
121250Z = Day 12, time 1250 UTC (Z = Zulu/UTC)
27015KT = Wind 270° at 15 knots
9999 = Visibility 10 km or more
FEW020 = Few clouds at 2,000 feet
15/08 = Temperature 15°C / Dewpoint 8°C
Q1012 = QNH 1012 hPa
NOSIG = No significant change expected (next 2 hours)

Wind

Wind is reported as a three-digit true direction followed by a two-digit speed. Gusts are appended with the letter G and the gust speed.

Code Format	Meaning	Example	Decoded
dddssKT	Direction (°) + Speed (KT)	27015KT	Wind 270° at 15 knots
dddssGggKT	Direction + Speed + Gust	27015G28KT	Wind 270° at 15, gusting 28 knots
00000KT	Calm wind	00000KT	Calm
VRBssKT	Variable direction	VRB03KT	Variable 3 knots
dddVddd	Variable sector (when >60°)	240V300	Wind variable between 240° and 300°
/////KT	Wind not observed	/////KT	Not reported

Note: Some countries and aerodromes use MPS (metres per second) instead of KT.

Visibility

Code	Meaning
9999	10 km or more
0000	Less than 50 m
1200	1,200 metres (4 digits = metres)
5000	5,000 metres
CAVOK	Ceiling And Visibility OK — visibility ≥10 km, no cloud below 5,000 ft/MSA, no significant weather

CAVOK means all of the following simultaneously: Visibility 10 km or more AND no cloud below 5,000 ft (or minimum sector altitude, whichever is higher) AND no cumulonimbus or towering cumulus AND no significant weather phenomena.

Weather Phenomena

Weather codes are constructed by combining an optional intensity prefix, an optional descriptor, and a precipitation or obscuration type. Multiple weather groups may appear in a single METAR.

Intensity Modifiers

Code	Meaning	Example
-	Light	-RA = light rain
(none)	Moderate	RA = moderate rain
+	Heavy	+TSRA = heavy thunderstorm with rain
VC	In the vicinity (within 8 km but not at aerodrome)	VCFG = fog in vicinity

Descriptor Codes

Code	Meaning
MI	Shallow
PR	Partial
BC	Patches
DR	Low Drifting
BL	Blowing
SH	Shower(s)
TS	Thunderstorm
FZ	Freezing

Precipitation

Code	Meaning
DZ	Drizzle
RA	Rain
SN	Snow
SG	Snow grains
IC	Ice crystals
PL	Ice pellets
GR	Hail (>5 mm)
GS	Small hail (<5 mm)
UP	Unknown precipitation (AUTO stations)

Obscuration

Code	Meaning
BR	Mist (visibility 1,000–5,000 m)
FG	Fog (visibility <1,000 m)
FU	Smoke
VA	Volcanic ash
DU	Widespread dust
SA	Sand
HZ	Haze
PY	Spray

Other Phenomena

Code	Meaning
PO	Dust/sand whirls
SQ	Squalls
FC	Funnel cloud (tornado or waterspout if + prefix)
SS	Sandstorm
DS	Duststorm

Cloud Cover

Cloud amounts are reported in oktas (eighths of sky covered). Heights are in hundreds of feet above aerodrome elevation.

Code	Coverage	Oktas	Notes
SKC	Sky Clear	0/8	Used at manual reporting stations
NCD	No Cloud Detected	0/8	AUTO station — no cloud detected by sensor
NSC	No Significant Cloud	0/8	No cloud below 5,000 ft, no CB/TCU; CAVOK conditions not met for another reason
FEW	Few	1–2	Not operationally significant in most contexts
SCT	Scattered	3–4	At or below MSA may affect operations
BKN	Broken	5–7	Sky considered "overcast" for IFR purposes in many jurisdictions
OVC	Overcast	8/8	Complete cloud cover
VV	Vertical Visibility	—	Sky obscured; figure is vertical visibility in hundreds of feet (e.g. `VV005` = 500 ft)

Format: FEW020 = Few clouds at 2,000 ft (height always in hundreds of feet above aerodrome elevation).
Special: BKN015CB = Broken cumulonimbus at 1,500 ft. Cumulonimbus (CB) and towering cumulus (TCU) are always reported explicitly when present, regardless of the cloud amount group.

Temperature & Dewpoint

Temperature and dewpoint are reported in degrees Celsius, separated by a slash. Sub-zero temperatures are prefixed with the letter M (minus).

# Format: TT/TD
15/08 = Temperature 15°C, Dewpoint 8°C
M02/M05 = Temperature -2°C, Dewpoint -5°C
00/M01 = Temperature 0°C, Dewpoint -1°C

The spread between temperature and dewpoint is a key indicator of fog risk. A spread of 2°C or less suggests fog is possible, especially when temperatures are falling toward the dewpoint overnight.

Pressure (QNH)

Code	Meaning	Example	Decoded
Q	QNH in hectopascals (hPa/mbar)	Q1012	QNH 1012 hPa
A	Altimeter setting in inches Hg (USA/Canada)	A2992	29.92 inHg

QNH is used to set the altimeter so that it reads altitude above mean sea level (AMSL). At or below the transition altitude, pilots set QNH; above the transition level they set 1013.25 hPa (standard) and read flight levels.

Remarks and Trend Forecasts

Trend information may be appended to a METAR, giving a short-term forecast for the next two hours. This is not a full forecast — only expected significant changes are noted.

Code	Meaning
NOSIG	No significant change expected in the next 2 hours
TEMPO	Temporary fluctuations — each lasting less than 60 minutes and occurring during less than half of the period
BECMG	Becoming — conditions expected to change at a steady or irregular rate within the indicated time period
PROB30	30% probability of the following conditions occurring
PROB40	40% probability of the following conditions occurring

TAF Format Overview

A TAF (Terminal Aerodrome Forecast) is issued by a meteorological office for an aerodrome, typically valid for 9, 24, or 30 hours. It uses the same element codes as METAR (wind, visibility, weather, cloud) but describes expected rather than observed conditions.

TAF EGLL 121100Z 1212/1318 27015KT 9999 FEW030
  TEMPO 1213/1216 27025G40KT 4000 RASN BKN015
  BECMG 1218/1220 29012KT
  PROB40 TEMPO 1222/1302 7000 -RA BKN025

TAF EGLL = TAF for London Heathrow
121100Z = Issued day 12 at 1100Z
1212/1318 = Valid from day 12 1200Z to day 13 1800Z
27015KT 9999 FEW030 = Base forecast: wind / visibility / cloud
TEMPO 1213/1216 = Temporary between 1300–1600Z on day 12
27025G40KT = wind 270°/25G40kt
4000 RASN BKN015 = 4 km vis, rain/snow, broken 1500 ft
BECMG 1218/1220 = Becoming between 1800–2000Z day 12
29012KT = wind changing to 290°/12kt
PROB40 TEMPO = 40% probability of temporary conditions

Time Group Format in TAFs

Time groups in TAFs use the format DDHH/DDHH where DD is the day of the month and HH is the hour in UTC. For example, 1212/1318 means from day 12 at 1200Z to day 13 at 1800Z.

AMD and COR

TAF AMD indicates an amended TAF replacing the previous one before its validity expires. TAF COR indicates a corrected TAF issued to fix an error in a previously distributed report.

SPECI — Special Observation Criteria

A SPECI (Special Report) is an unscheduled METAR issued when significant weather changes occur between routine hourly reports. The following 12 criteria trigger a SPECI:

#	Criterion	Threshold
1	Wind shift	Direction change ≥45° in <15 min with sustained wind ≥10 kt throughout
2	Visibility	Increases or decreases through: 3 SM, 2 SM, 1 SM, or the lowest published IAP minimum (½ SM if none published)
3	RVR	Increases or decreases through 2,400 ft
4	Tornadic activity	Tornado, funnel cloud, or waterspout begins or ends
5	Thunderstorm	Thunderstorm begins or ends
6	Precipitation type	Hail begins/ends; freezing precip begins/ends/changes intensity; ice pellets begin/end/change intensity; snow begins/ends/changes intensity
7	Squall	Squall occurs (sudden onset, wind increases ≥16 kt, sustained ≥22 kt for ≥1 min)
8	Ceiling	Increases or decreases through: 3,000 ft, 1,500 ft, 1,000 ft, 500 ft, or lowest published IAP minimum
9	New ceiling below 1,000 ft	A layer below 1,000 ft appears where none was before
10	Volcanic eruption	Volcanic eruption occurs
11	Aircraft mishap	Aircraft mishap at or near the station
12	Miscellaneous	Other operationally significant events as determined locally

NOSPECI — some stations do not issue SPECIs. When coded in remarks, it indicates this station does not take special observations.

FIRST / LAST — coded in remarks when a station resumes observations after a break (FIRST) or when it is the final observation before a coverage gap (LAST).

Wind Observation Standards

Concept	Standard / Threshold
Averaging period	2-minute mean for both direction and speed
Variable direction (VRB)	Speed ≤6 kt with variable direction; coded as `VRB03KT`
Variable direction group	Speed >6 kt with direction varying ≥60°; coded as main direction then `180V240` suffix
Gust (G)	Rapid fluctuations with ≥10 kt variation between peak and lull; peak speed appended: `27018G32KT`
Squall (SQ)	Sudden onset; speed increases ≥16 kt and is sustained ≥22 kt for ≥1 minute. Coded as `SQ` in present weather.
Peak wind RMK	When peak gust >25 kt occurs since last METAR, coded in remarks as `PK WND 27041/1352`
Wind shift RMK	Direction change ≥45° in <15 min with ≥10 kt; coded as `WSHFT 1341` (time of shift)

Visibility Standards

Concept	Detail
Prevailing visibility	Greatest visibility equalled or exceeded throughout at least half the horizon circle; this is what is reported in the body of the METAR
Sector visibility	Visibility in a specific direction; reported in remarks when differing from prevailing
Tower visibility	Visibility from ATC tower; coded in remarks as `TWR VIS 1 1/2`
Surface visibility	Visibility from ground level; coded in remarks as `SFC VIS 3/4`
Second-location visibility	Visibility at a runway threshold: `VIS 3/4 RWY11`
Variable prevailing vis	When prevailing visibility varies ≥½ SM and is <3 SM: `VIS 1/2V1 1/2` in remarks
M (less than) prefix	Automated station reports visibility below lowest reportable value: `M1/4SM`

US Reportable Visibility Values (ASOS)

Automated observations report the following values (SM): M1/4, 1/4, 1/2, 3/4, 1, 1 1/4, 1 1/2, 1 3/4, 2, 2 1/2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30+. Manual stations report additional intermediate values.

RVR — Runway Visual Range (Detail)

Parameter	Detail
Reporting trigger	RVR is reported when prevailing visibility is ≤1 SM or any RVR value is ≤6,000 ft
Format	`R28L/2400FT` = Runway 28 Left, RVR 2,400 ft
M (below minimum)	`R06R/M0600FT` = RVR is below the lowest reportable value of 600 ft
P (above maximum)	`R06R/P6000FT` = RVR exceeds the highest reportable value of 6,000 ft
Variable RVR	`R28L/2000V4000FT` = RVR varying between 2,000 and 4,000 ft
Multiple runways	Up to 4 runway RVR values may be reported from automated stations

RVR Reporting Increments

RVR Range	Increment
600–1,000 ft	100 ft
1,000–3,000 ft	200 ft
3,000–6,000 ft	500 ft

Present Weather — Intensity & Definitions

Fog vs Mist — Exact Boundary

Code	Phenomenon	Visibility
FG	Fog	<5/8 SM (<1,000 m)
BR	Mist	5/8 SM up to 6 SM (1,000 m to <7 km)
HZ	Haze	Visibility reduced by dry particles; no surface moisture

Precipitation Intensity

Intensity Prefix	Rain / Ice Pellets (rate)	Snow / Drizzle (by visibility)
Light (−)	≤0.10 in/hr	Visibility >½ SM
Moderate (no prefix)	0.11–0.30 in/hr	Visibility >¼ SM to ½ SM
Heavy (+)	>0.30 in/hr	Visibility ≤¼ SM

Descriptor Usage Rules

Descriptors can only combine with specific phenomena:

Descriptor	Used only with
MI	`FG` only (shallow fog — vertical extent <6 ft)
PR	`FG` only (partial fog — coverage <half the aerodrome)
BC	`FG` only (patchy fog — irregular, <20 ft vertical extent)
DR	`DU SA SN` (low drifting — below 6 ft)
BL	`DU SA SN PY` (blowing — 6 ft or higher)
SH	`RA SN PL GS GR` (showers — not used with TS)
TS	Any precipitation; not coded with SH
FZ	`FG DZ RA` only (freezing)

Fog Subtype Vertical Extent

Code	Type	Vertical Extent
MIFG	Shallow fog	<6 ft above surface
PRFG	Partial fog	Covers <half the aerodrome
BCFG	Patches of fog	≥6 ft but <20 ft; irregular distribution
FG	Fog	≥20 ft; uniform; vis <5/8 SM

Unknown Precipitation

UP — automated stations code precipitation type as unknown when the sensor cannot identify it. This code is only used on automated (ASOS) reports.

Sky Condition — Reporting Standards

Cloud Height Reporting Increments

Height Range	Reported to Nearest
Surface to 5,000 ft	100 ft
5,001 to 10,000 ft	500 ft
Above 10,000 ft	1,000 ft

Variable Ceiling Criteria

A ceiling is reported as variable (in RMK) when it fluctuates by the following amounts:

Ceiling Height	Variation Required to Report Variable
≤1,000 ft	≥200 ft variation
1,001–2,000 ft	≥400 ft variation
2,001–3,000 ft	≥500 ft variation

Layer Reporting Priority

When more than 6 cloud layers exist, the following are selected for reporting: the first layer that makes a ceiling (BKN or OVC) at or below 5,000 ft; the highest layer of each coverage group; and cumulonimbus or towering cumulus regardless of amount. CB and TCU are always appended to the layer amount (SCT015TCU, BKN030CB).

Significant Cloud Types (Sky Condition Suffixes)

Code	Cloud Type	Notes
CB	Cumulonimbus	Thunderstorm cloud — always reported
TCU	Towering Cumulus	Pre-CB stage — always reported
CBMAM	Cumulonimbus Mammatus	CB with mammatus clouds (remarks)
ACSL	Altocumulus Standing Lenticular	Wave cloud; mountain wave indicator (remarks)
CCSL	Cirrocumulus Standing Lenticular	High-level wave cloud (remarks)
SCSL	Stratocumulus Standing Lenticular	Low-level wave cloud (remarks)
APRNT ROTOR CLD	Apparent Rotor Cloud	Severe turbulence indicator below mountain wave (remarks)

METAR Remarks (RMK) — Complete Reference

The RMK group follows the body of US METARs and contains additional operational information not encoded in the standard fields. Remarks appear in a defined order.

Station Type

Code	Meaning
AO1	Automated station without precipitation discriminator (cannot distinguish rain from snow)
AO2	Automated station with precipitation discriminator
TESTM	Non-commissioned ASOS test report; not for operational use

Peak Wind and Wind Shift

Code	Format	Example / Decoded
PK WND	`PK WND dddff(f)/(hh)mm`	`PK WND 27041/1352` = Peak wind 270° at 41 kt at 1352Z
WSHFT	`WSHFT (hh)mm`	`WSHFT 1341` = Wind shift occurred at 1341Z
WSHFT FROPA	With frontal passage	`WSHFT 1341 FROPA` = Wind shift due to frontal passage at 1341Z

Visibility Remarks

Code	Example	Meaning
TWR VIS	`TWR VIS 1 1/2`	Tower visibility 1½ SM (differs from surface)
SFC VIS	`SFC VIS 3/4`	Surface visibility ¾ SM (differs from tower)
VIS v1Vv2	`VIS 1/2V1 1/2`	Prevailing visibility variable between ½ and 1½ SM
VIS [dist] [loc]	`VIS 3/4 RWY11`	Visibility ¾ SM at runway 11 threshold

Lightning

Element	Codes	Meaning
Frequency	`OCNL` / `FRQ` / `CONS`	Occasional (<1/min) / Frequent (1–6/min) / Continuous (>6/min)
Type	`CA` / `CC` / `CG` / `IC`	Cloud-Air / Cloud-Cloud / Cloud-Ground / In-Cloud
Location	`N NE E SE S SW W NW OVHD VC DSNT`	Direction or overhead or vicinity or distant

RMK OCNL LTGCG NE = Occasional cloud-to-ground lightning to the northeast RMK FRQ LTGIC OVHD = Frequent in-cloud lightning overhead RMK CONS LTGCA NW = Continuous cloud-air lightning to the northwest

Lightning distance automation (FAA/NWS): ASOS automated detection follows these rules:

Lightning within 5 NM → TS coded in the METAR body (present weather)
Lightning 5–10 NM → VCTS coded in the METAR body (vicinity thunderstorm)
Lightning 10–30 NM → coded in remarks only (e.g. DSNT LTGCG NE)

Precipitation Begin / End Times

Format	Example	Decoded
`wxBhhmmEhhmm`	`RAB07E30`	Rain began at :07, ended at :30 (past hour)
`wxBhhmm`	`SNB1420`	Snow began at 1420Z
`TSBhhmmEhhmm`	`TSB0159E30`	Thunderstorm began at 0159Z, ended at :30
Multiple types	`RAB07E30SNB20E55`	Rain began :07 ended :30; snow began :20 ended :55

Thunderstorm Location and Movement

TS SE MOV NE = Thunderstorm to the southeast, moving northeast TS OHD MOV NE = Thunderstorm overhead, moving northeast VIRGA SW = Virga (precipitation not reaching ground) to the southwest GR LESS THAN 1/4 = Hailstones less than ¼ inch diameter

Variable Ceiling

Format	Example	Decoded
`CIG hhhVhhh`	`CIG 013V017`	Ceiling variable between 1,300 and 1,700 ft
`CIG hhh [LOC]`	`CIG 017 RWY11`	Ceiling 1,700 ft at runway 11 (second ceilometer)

Obscurations in Sky Condition

When the sky is obscured by a phenomenon on the surface (fog, smoke, dust), the obscuring layer is coded in remarks showing what is causing the obscuration and at what height:

RMK FG SCT000 = Sky obscured (VV in body); fog layer at surface (scattered by ceilometer) RMK FU BKN020 = Sky partially obscured by smoke up to 2,000 ft

Variable Sky Condition

RMK BKN014 V OVC = Broken layer at 1,400 ft varying to overcast

Pressure Remarks

Code	Meaning	Criteria
PRESFR	Pressure Falling Rapidly	Rate ≥0.06 inHg/hr and total fall ≥0.02 inHg
PRESRR	Pressure Rising Rapidly	Rate ≥0.06 inHg/hr and total rise ≥0.02 inHg
SLPppp	Sea-Level Pressure	`SLP045` → prefix 9 or 10, then tenths: = 1004.5 hPa. If first digit <5, prefix 10; if ≥5, prefix 9. So SLP971 = 997.1 hPa; SLP045 = 1004.5 hPa
SLPNO	SLP not available	Sensor failure

Sensor Status Indicators

Code	Meaning
RVRNO	RVR system not operating
PWINO	Present weather identifier not operating (cannot detect precip type)
PNO	Precipitation accumulation sensor not operating
FZRANO	Freezing rain sensor not operating
TSNO	Lightning sensor not operating
VISNO [LOC]	Visibility sensor at specified location not operating (e.g. `VISNO RWY06`)
CHINO [LOC]	Ceiling sensor at specified location not operating (e.g. `CHINO RWY11`)
$	Station requires maintenance — automated station is reporting but a sensor needs attention

Additive Data Groups (Remarks)

Additive data groups provide supplementary weather data in numeric coded form, appended after the standard remarks.

Group	Format	Example	Decoded
Hourly precip	`Prrrr`	`P0015`	0.15 in precipitation in last hour
3/6-hr precip	`6RRRR`	`60023`	0.23 in precipitation in last 6 hours (or 3 hrs at 00/06/12/18Z)
24-hr precip	`7RRRR`	`70098`	0.98 in precipitation in last 24 hours
Snow depth	`4/sss`	`4/013`	Snow depth 13 inches on ground
Snow water equiv	`933RRR`	`933025`	2.5 in water equivalent of snow on ground
Hourly temp/dew	`TsnT'T'T'snT'dT'dT'd`	`T00640036`	Temp +6.4°C, Dewpoint +3.6°C (to tenths)
6-hr max temp	`1snTxTxTx`	`10066`	6-hr max temperature +6.6°C
6-hr min temp	`2snTnTnTn`	`21012`	6-hr min temperature −1.2°C
24-hr max/min temp	`4snTxTxTxsnTnTnTn`	`400461006`	24-hr max +4.6°C, min −0.6°C
Sunshine duration	`98mmm`	`98060`	60 minutes of sunshine in last hour

Pressure Tendency Group (5appp)

Coded at 00/06/12/18Z observations. Format: 5appp where a is the characteristic (0–8) and ppp is the 3-hour pressure change in tenths of hPa.

Code (a)	Pressure Characteristic
0	Increasing, then decreasing — pressure now equal to or higher than 3 hr ago
1	Increasing, then steady; or increasing, then increasing more slowly
2	Increasing steadily or unsteadily
3	Decreasing or steady, then increasing; or increasing more rapidly
4	Steady — same as 3 hours ago
5	Decreasing, then increasing — pressure now equal to or lower than 3 hr ago
6	Decreasing, then steady; or decreasing, then decreasing more slowly
7	Decreasing steadily or unsteadily
8	Steady or increasing, then decreasing; or decreasing more rapidly

58033 = Pressure characteristic 8 (steady then decreasing rapidly), change 3.3 hPa over 3 hours

TAF — Additional Codes & Standards

Additional TAF Change Groups

Code	Meaning	Notes
FM	From — conditions change completely at the stated time	`FM1800` = From 1800Z, all previous groups cancelled
TEMPO	Temporary — each fluctuation lasts <60 min, total <half the period	Must specify time group
PROB30	30% probability of conditions in next group	NWS domestic TAFs use PROB30 only (not PROB40)
BECMG	Becoming — gradual change over period	Not used in NWS domestic US TAFs; used internationally (ICAO)
NSW	No Significant Weather	Used in TAF to cancel previously forecast weather phenomena; means weather has ended

Low-Level Wind Shear in TAF (WS Group)

Non-convective low-level wind shear below 2,000 ft is coded as a WS group:

WS020/27055KT WS = Wind shear group 020 = Height 2,000 ft (in hundreds of feet) / = Separator 270 = Wind direction at that height 55KT = Wind speed at that height

Trigger criteria: based on PIREPs, surface observations, and model data indicating non-convective LLWS within 2 hours of valid time.

CAVOK — Conditions & Limitations

CAVOK (Ceiling And Visibility OK) replaces visibility, weather, and cloud groups when ALL of the following are met:

Visibility ≥10 km (6 SM)
No cloud below 5,000 ft (1,500 m) or below the highest MSA/MEA, whichever is higher
No cumulonimbus (CB) at any level
No significant weather phenomena

Note: NWS domestic US TAFs do NOT use CAVOK — they report 9999 for visibility and SKC or FEW for cloud instead. CAVOK is used in ICAO TAFs and international METARs.

Fog and Mist Encoding in TAF

Visibility in TAF	Code
5/8 SM to 6 SM with surface moisture	BR
<5/8 SM	FG
>6 SM (US TAF)	P6SM

Multiple Precipitation Types — Encoding Order

When multiple precipitation types occur simultaneously, freezing types are coded first regardless of intensity (e.g., FZRASN = freezing rain and snow).

Vicinity (VC) Codes in TAF

Code	Meaning
VCFG	Fog in the vicinity (5–10 SM from runway complex)
VCSH	Showers in the vicinity
VCTS	Thunderstorm in the vicinity

TAF Issuance Schedule (NWS)

Issuance Time (UTC)	Valid Period (standard sites)
0000Z	0000–2400Z (24 hr)
0600Z	0600–0600Z +1 (24 hr)
1200Z	1200–1200Z +1 (24 hr)
1800Z	1800–1800Z +1 (24 hr)

Select high-traffic airports issue 30-hour TAFs. AMD NOT SKED is appended when no amendment is planned outside normal issuance times (e.g. part-time stations). COR indicates a correction to a previously issued TAF.

Vicinity Definition — METAR vs TAF

Context	Vicinity (VC) Means
US METAR	5–10 SM from the point of observation
US TAF	5–10 SM from the runway complex
ICAO (international)	Within 8,000 m of the aerodrome reference point