Velocity fields

Three layer types draw a field of velocities — wind, currents, any flow given as a vector at each place. Two are this plugin's and are built from a query. The third is kepler's own, which the panel steps aside for when it has built its own.

flowfield

SELECT ts AS "time", lat, lon, speed, direction
FROM wind_grid;

Named Streamlines in kepler's layer menu. Built automatically from a position and a velocity, with no trip id, when the rows lie on a regular lattice. Animated streamlines traced through the grid; the dashboard's clock chooses which hour of a forecast is drawn.

See Wind and other velocity fields.

vectorfield

Arrows or wind barbs over the same grid, one mark per sample. Never guessed: switch a Streamlines layer's type, or add it from Add Layer. See Arrows and wind barbs.

flowField

SELECT lat, lon, u, v
FROM station_winds;

kepler's own streamlines layer, Flow Field in the layer menu, new in kepler 3.3. Like Streamlines it draws animated trails through a field; unlike it, it builds the field from whatever rows it is given, scattered or not. The image shows 400 scattered rows.

When you get it

kepler creates it by itself for columns named exactly u and v beside a position. What the panel does with it depends on the rows:

The rows are	What the map gets
A regular lattice, no trip id	The panel's Streamlines. kepler's Flow Field and Point layer are removed.
Scattered — stations, buoys — with u and v	kepler's Flow Field and a Point layer. The panel builds no Streamlines.
Scattered, with speed and direction or other names	Points only. Add the Flow Field by hand.

Removing it over a lattice is deliberate: the two layers would draw two sets of lines over one grid, the second on a clock of its own. It stays in Add Layer for anyone who wants it.

To add it by hand, choose Add Layer → Flow Field, then a column mode and the columns.

What it needs

A position, as Lat / Lng or as a GeoJSON column (each feature's centroid), and one of three column modes:

Column mode	Required	Optional
U / V components	u (eastward), v (northward)	altitude
Speed / direction	speed, direction — the bearing it comes from	altitude
Altitude (downhill)	altitude, a value whose downhill slope is the flow	—

The optional altitude lifts the lines in 3D, scaled by Elevation Multiplier.

How it builds the field

Rows that already lie on a regular spacing are used as the grid itself, as long as neither side has more cells than Grid Resolution. Any other rows are dropped into a grid of that many cells along the longer side, and the cells holding several rows take their average. Smoothing then blurs the field over that many neighbouring cells, which also fills empty ones; at 0 the raw samples are used and empty cells stay empty.

So between stations what you see is the blur, not a measurement. The Debug Grid switch draws the grid and marks the cells that hold data.

Settings

Setting	Default	What it does
Lines Per Screen	7500	How many streamlines to aim for in the current view
Zoom Response	0.25	0 keeps the line count as you zoom; 1 keeps their spacing on the ground
Stroke Width	3	Line width in pixels
Color By Speed	off	Colour by speed with the palette, or one colour
Opacity	0.55
Trail Length	22	Visible trail, as a percent of the cycle
Cycle	20	Seconds for one loop of the animation
Line Lifetime	0.55	How long a trail stays, as a fraction of the cycle
Seamless Loop	on	Repeats the trails so they do not pop off at the end of the cycle
Smoothing	4	Blur radius, in grid cells
Grid Resolution	64	Grid cells along the longer side of the field
Elevation Multiplier	1	Scales the altitude column's height
Debug Grid	off	Draws the grid

Next to Streamlines

	Streamlines (flowfield)	Flow Field (flowField)
Rows	a regular lattice; anything else draws nothing	anything; binned, averaged and smoothed
Built from	speed and direction, or u and v, under several names	u and v exactly; other modes by hand
Hours of a forecast	the map's time filter picks the latest hour in its window	no time input: every row it receives goes into one grid
Direction	where it comes from or where it goes, set on the layer	where it comes from
From a single value	downhill, uphill or along the contours	downhill
Position	latitude and longitude	latitude and longitude, or a GeoJSON column
Arrows instead of lines	switch to vectorfield	—

Use Streamlines for gridded model output — forecasts, reanalyses, ocean models — and whenever the query carries several hours. Use kepler's Flow Field for observations that are not on a grid, and read it as an interpolation.

Backed by deck.gl's TripsLayer, which kepler animates on a loop of its own.