raster

Much of the data under OS OpenData. is available in a "raster" format, with image files representing tiles.

One such dataset is the OS VectorMap District set, which this package was developed against, though it may work with other raster datasets, with no or minimal modification - I haven't tested that.

This package provides a way to work with a raster dataset, querying images and pixel locations corresponding to grid references.

To use it, download (all, or some of) the OS VectorMap District data in "GeoTIFF Full Colour" format.

package main

import (
	"fmt"
	"image"
	"image/draw"
	"image/png"
	"os"

	"github.com/usedbytes/osgrid"
	"github.com/usedbytes/osgrid/osdata/raster"
)

func main() {
	if len(os.Args) != 2 {
		fmt.Println("Provide path to raster data as only argument")
		return
	}

	db, err := raster.OpenDatabase(os.Args[1], 10 * osgrid.Kilometre)
	if err != nil {
		panic(err)
	}

	summit, _ := osgrid.ParseGridRef("SH 60986 54375")

	tile, _ := db.GetImageTile(summit)

	x, y, _ := tile.GetPixelCoord(summit)

	out := image.NewRGBA(image.Rect(0, 0, 500, 500))

	draw.Draw(out, out.Bounds(), tile.GetImage(), image.Pt(x - 250, y - 250), draw.Over)

	f, err := os.Create("out.png")
	if err != nil {
		panic(err)
	}
	defer f.Close()

	png.Encode(f, out)
}

A tile cache is used (with 16 entries by default), storing the parsed data for the 16 most-recently-used tiles so that queries which are geographically close to each other are fast, and to ensure memory usage doesn't grow unbounded.

The GenerateSurface() function in lib/geometry provides the functionality to query elevation data for a rectangular region.

Known Issues

The golang.org/x/image/tiff package which is used for decoding the TIFF images has an issue where it throws "unexpected EOF" for some images. For example tile NO08 exhibits this problem.

I've posted a patch which appears to fix it: https://github.com/golang/go/issues/30827#issuecomment-774469551

I probably should either make a vendored version of that module and pin it in this package, or try out a different TIFF decoder package.