GridTiler

Produce tiled grids in tiled grid format for web mapping applications such as GridViz.

Installation

• Install Node.js, version >=14
• Run npm install gridtiler -g (with sudo for Linux users)

gridtiler command is now available.

Documentation

Run gridtiler --help to show the help, or see there:

Parameter	Required	Description	Default value
-i, --input	X	Input CSV file. One row per cell. The file is expected to include two x and y columns for the coordinates of the lower left corner. If not, see positionFunction parameter.
-o, --output		output folder where to produce the tiled grid.	"out/"
-s, --columns		The columns to keep, as comma separated list. If not specified, all columns are kept.	undefined
-e, --outencodings		output encodings format: 'csv' or 'parquet'.	"csv"
-r, --resolutionGeo	X	The grid resolution, that is the size of a grid cell in the CRS unit.
-t, --tileSizeCell		The size of the tile in number of cells.	128
-x, --originPointX		The X coordinate of the tiling scheme origin point (bottom left).	0
-y, --originPointY		The Y coordinate of the tiling scheme origin point (bottom left).	0
-a, --aggregationFactor		In case there is the need for aggregating the cells to lower resolution, specify this parameter. The resolution of the aggregated grid will be this parameter multiplied by the input resolution resolutionGeo.	undefined
-R, --aggregationRounding		When aggregating, the number of decimals of the aggregated figure.	6
-p, --positionFunction		A javascript function body returning the position of an input cell c as a {x,y} object.	"return { x: c.x, y: c.y };"
-f, --filterFunction		A javascript function body specifying if an input cell should be filtered or kept. Return true to keep, false to filter out.	undefined
-m, --preFunction		A javascript function body modifying an input cell c at the beginning of the process, before the aggregation and tiling. This may be used for example to remove unecessary columns, or computing new ones from the combination of others. This function applies after filtering.	undefined
-n, --postFunction		A javascript function body modifying an input cell c after the aggregation and before the tiling. This may be used to alter cell data after the aggregation.	undefined
-d, --delim		The CSV delimiter.	","
-c, --crs		EPSG code of the grid Coordinate Reference System.	""
-V, --version		Show version number.
-h, --help		Show the help.

Usage

Default

The default input format is a CSV file with the following specifications:

• One grid cell per row.
• A x and y columns with the coordinates of the cell bottom left corner position in the grid CRS.
• Other columns with data of the grid cells, as many as necessary, numerical or not.
• The order of the columns does not matter. For example, the y column could be the last one.

Here is an example, for a grid of 5 cells of resolution 10 with two data columns pop (numerical) and type (text):

x,y,pop,type
1000,2000,54654,A
1010,2000,4554,B
5000,2000,6434,C
4150,2560,43,B
4160,2560,43,A

If this data is stored in a grid.csv file, simply run gridtiler -i grid.csv -r 10 in the folder where the grid.csv file is located to produce the tiled grid in a out/ folder.

Since the grid is located north and east of point (1000,2000), this point could be used as an origin point. Run gridtiler -i grid.csv -r 10 -x 1000 -y 2000 to adapt this.

Custom cell position

When the input data does not provide explicit x and y columns for the bottom left position of each cell, the positionFunction parameter can be used to derive this position from other cell data. This parameter is a javascript function which returns the bottom left position of a cell. By default, this function body is return { x: c.x, y: c.y }; which simply returns the x and y column values of a cell c.

Examples:

• If the bottom left coordinates are in two posX and posY columns, use: --positionFunction "return { x: c.posX, y: c.posY };"
• If the cells position is the position of its center in two columns xCentre and yCentre, and the grid resolution is 1000, use: --positionFunction "return { x: c.xCentre - 500, y: c.yCentre - 500 };" which translates the centre position toward the bottom left corner.
• If the cell is described by its INSPIRE identifier, see below.

Dealing with INSPIRE identifier

If the cell is described with an INSPIRE identifier (such as CRS3035RES5000mN4585000E5265000) in a GRD_ID column, use the following parameter to extract the cell position: --positionFunction "const a=c.GRD_ID.split('N')[1].split('E');return {x:a[1],y:a[0]};". This GRD_ID column may then be removed in the output tiles with --postFunction "delete c.GRD_ID"

Example: With European population grids downloaded from Eurostat grids page, use gridtiler -i pop_5000m.csv -r 5000 --positionFunction "const a=c.GRD_ID.split('N')[1].split('E');return {x:a[1],y:a[0]};" --postFunction "delete c.GRD_ID"

Aggregation

Gridtiler offers the possibility to aggregate grid cells to lower resolutions, on-the-fly, using the -a, --aggregationFactor parameter. The resolution of the target grid will be this parameter time the input grid resolution specified under -r, --resolutionGeo parameter. The tiling then applies on the aggregated grid.

Example: The command gridtiler -i pop_5000m.csv -r 5000 -a 4 aggregates the grid pop_5000m.csv from the input resolution 5000 to the resolution 5000 x 4 = 20000.

Note that currently grids with only numerical columns can be aggregated, and the aggregation is computed as a sum. This aggregation may be extended to support other types of aggregations such as average, median, min, max, near or mode (for non-numerical values).

Tiled multi-resolution grid

Computing aggregations with consecutive resolutions makes possible to produce a pyramid of multi-resolution tiled grids from a single high-resolution input grid. Such multi-resolution may then be used for efficient multi-scale visualisation for example with GridViz.

For example, these commands produce tiled grid at various resolutions from 1km to 100km from a single input 1km resolution dataset pop_2018_1km:

gridtiler -i pop_2018_1km.csv -r 1000 -a 1 -o 1000m/
gridtiler -i pop_2018_1km.csv -r 1000 -a 2 -o 2000m/
gridtiler -i pop_2018_1km.csv -r 1000 -a 5 -o 5000m/
gridtiler -i pop_2018_1km.csv -r 1000 -a 10 -o 10000m/
gridtiler -i pop_2018_1km.csv -r 1000 -a 20 -o 20000m/
gridtiler -i pop_2018_1km.csv -r 1000 -a 50 -o 50000m/
gridtiler -i pop_2018_1km.csv -r 1000 -a 100 -o 100000m/

Filtering and modifying

• Use the parameter filterFunction to specify if a cell should be filtered or kept. Return true to keep, false to filter out.
• Use preFunction to modify the cells at the beginning of the process, before the aggregation and tiling. This may be used for example to remove unecessary columns, or computing new ones from the combination of others. This function applies after filtering.
• Use postFunction to modify the cells after the aggregation and before the tiling. This may be used to alter cell data after the aggregation.

For example, to select only the cells with a type value A or B, multiply the pop value by 1000 and add a new column new with the value 100, run:

gridtiler -i grid.csv -r 10 --filterFunction "return c.type==='A' || c.type==='C'" --preFunction "c.pop*=1000;c.new=100"

Other resources

Grid tiling in:

• Python
• Java

About

contributors
version	See npm
status	Since 2023
license	EUPL 1.2

Support and contribution

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