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ascii-grid
Advanced tools
Identify, Read, and Write an ARC/INFO ASCII Grid
I do a lot of client-side geoprocessing (see geoblaze) and wanted to add support for .asc files. When I encountered large .asc files, I quickly ran out of memory because I was trying to load the whole file into memory. This package was created to make it easy to read specific areas of an ASCII Grid in a memory-safe way and prevent my laptop from overheating.
npm install ascii-grid
https://ascii-grid.github.io/ascii-grid/
isAsciiGrid identifies ASCII GRID files in the following formats: ArrayBuffer, Buffer, DataView, Promise, String, and Uint8Array
const isAsciiGrid = require("ascii-grid/src/is-ascii-grid");
const buffer = readFileSync('./test_data/michigan_lld/michigan_lld.asc');
isAsciiGrid({ data: buffer, debug: false });
// true
const parseAsciiGridMeta = require("ascii-grid/src/parse-ascii-grid-meta");
const buffer = readFileSync('./test_data/michigan_lld/michigan_lld.asc');
const metadata = parseAsciiGridMeta({
data: buffer,
debug: false,
cache: true, // caches metadata, but increases memory usage,
raw: false // true returns numbers as raw strings
});
/*
{
ncols: 4201,
nrows: 5365,
xllcenter: -88,
yllcenter: 41.62,
cellsize: 0.0008333333333,
nodata_value: -9999,
last_metadata_line: 5,
last_metadata_byte: 95
}
*/
const parseAsciiGridData = require("ascii-grid/src/parse-ascii-grid-data");
const result = await parseAsciiGridData({
data: buffer,
debug: true,
cache: true, // caches metadata, but increases memory usage
meta // optionally pass in metadata from parseAsciiGridMeta
});
/*
result is an object with a values array that holds
two-dimensional array of rows with pixel values
{
values:
[
[55.874908, 57.874924, 58.874939, ...], // first row
[62.875015, 63.875031, 62.875046, ...],
[62.875122, 64.875137, 63.875168, ...],
.
.
.
[52.875671, 50.875702, 51.875717, ...], // last row
]
}
*/
You can specify a bounding box to read from by specifying the zero-based index values of the first and last row, and first and last column for each row
const parseAsciiGridData = require("ascii-grid/src/parse-ascii-grid-data");
const result = await parseAsciiGridData({
data: buffer,
debug: true,
start_column: 2, // start reading from the third column
end_column: 10, // read through the eleventh column
start_row: 1, // skip the first row
end_row: undefined // read through the end
});
/*
result's values array is the size of the bbox.
each row has a length of end_column - start_column + 1
{
values:
[
[62.875046, ...], // first row starting with index of start_row
[63.875168, ...],
.
.
.
[51.875717, ...], // last row
]
}
*/
Sometimes you may require the data to be returned in a one-dimensional flat array instead of split up into rows. To do so, set flat to true like below
const parseAsciiGridData = require("ascii-grid/src/parse-ascii-grid-data");
const result = await parseAsciiGridData({
data: buffer,
flat: true
});
/*
result's values array is as long as width * height
{
values: [55.874908, 57.874924, 58.874939, ...]
}
*/
If you don't want to save a large array of all the grid points, but rather iterate over the points with a callback, see below:
const forEachAsciiGridPoint = require("ascii-grid/src/for-each-ascii-grid-point");
forEachAsciiGridPoint({
data: buffer,
callback: ({ c, r, num }) => {
console.log("row index is", r);
console.log("column index is", c);
console.log("value is", num);
}
});
You can calculate statistics for the ASCII grid. Calculations are made by iterating over the grid points in a memory-aware way, avoiding loading the whole grid into memory. It uses calc-stats for the calculations.
const calcAsciiGridStats = require("ascii-grid/calc-ascii-grid-stats");
const stats = calcAsciiGridStats({ data: buffer });
/*
stats is
{
median: 24.926056,
min: -275.890015,
max: 351.943481,
sum: 304535462.0868404,
mean: 13.685328213781924,
modes: [6.894897],
mode: 6.894897,
histogram: {
"23.1291283": {
n: 23.1291283, // the actual value in numerical format
ct: 82 // number of times that n appears
},
.
.
.
}
}
*/
const writeAsciiGrid = require("ascii-grid/src/write-ascii-grid");
const result = writeAsciiGrid({
// you can also pass in the data as one flattened row
data: [
[123, 456, ...], // first row
[789, 1011, ...], // second row
// ...
],
// number of columns
ncols: 4201,
// number of rows
nrows: 5365,
// alternatively you can use xllcorner and yllcorner
xllcenter: -88,
yllcenter: 41.62,
cellsize: 0.0008333333333,
/****** THE FOLLOWING IS OPTIONAL ******/
// throw an error if necessary metadata is missing
// default is true
strict: false,
// set to 1+ for increased logging
// default is 0
debug_level: 2,
// whether to include a newline at the end of the file
// default is true
trailing_newline: false,
// the number of digits after the decimal
// doesn't apply to nodata_values
// default is no rounding.
// for more explanation of the rounding please see the documentation for JavaScript's toFixed function [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Number/toFixed)
fixed_digits: 6,
nodata_value: -9999
});
You can calculate the bounding box of the ASCII Grid using floating-point arithmetic.
import calcAsciiGridBoundingBox from "ascii-grid/src/calc-ascii-grid-bounding-box";
calcAsciiGridBoundingBox({
data, // array buffer, buffer, or string representing ascii grid file
// if no meta object is provided,
// calcAsciiGridBoundingBox will internally call parseAsciiGridMeta
meta,
// max_read_length is an optional parameter passed to parseAsciiGridMeta
max_read_length
});
[491501, 2556440, 594634.0933000001, 2645315.3392]
If precision is more important than speed, you can calculate the precise bounding box of the the ASCII Grid avoiding floating-point arithmetic errors.
import calcAsciiGridPreciseBoundingBox from "ascii-grid/src/calc-ascii-grid-precise-bounding-box";
calcAsciiGridPreciseBoundingBox({
data, // required
meta, // optional
max_read_length // optional
})
["491501", "2556440", "594634.0933", "2645315.3392"]
FAQs
Identify and Read an ARC/INFO ASCII Grid
The npm package ascii-grid receives a total of 20 weekly downloads. As such, ascii-grid popularity was classified as not popular.
We found that ascii-grid demonstrated a not healthy version release cadence and project activity because the last version was released a year ago. It has 1 open source maintainer collaborating on the project.
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