.asc format encodes raster data. It has a header followed by rows of numbers giving the values for the region it describes.
It is often used to incode LIDAR height data for a region. These files are created by processing the raw readings from flights. It can be inspected with a normal text editor but you will want to process it to gain understanding from it.
It describes heights of a two dimensional set of cells. Each file has the coordinate of either the lower-left or centre of the region being described, the size of each cell in meters, and the number of rows and columns. So for example a cell size of 20 and nrows and ncols of 50 would describe a 1km region. Cells with unknown values have a specific distinct value defined at the top of the file, often -9999.
(Actually it can encode other data too, but it's often LIDAR...)
What is LIDAR?
- http://blogs.qub.ac.uk/gis/2015/10/15/aerial-light-detection-and-ranging-lidar/ - blog post explaining this in detail.
DSM and DTM
These terms are used frequently to describe the contents of a .asc file. DSM is the surface, which means the highest reading, so includes the shapes of trees and buildings. DTM describes only the terrain with other features removed using various methods, including taking the lowest reading rather than the highest, and using software to eliminate things that appear to be structures. Subtracting the terrain from the surface reading will give the height of features such as trees and buildings.
Data is taken by flights and is usually from a few years ago, rather than very recent.
Make sure you check that you fully understand the projection of the data and anything you are merging it with. If your results are more than 10m out of alignment then you probably have an incorrect projection.
If you want to consdir the value of a cell at a point then it should be the center of the cell, not the bottom left corner.
Description of file format
- https://github.com/ffisegydd/pylidar - Python Library
- http://www.qgis.org/en/site/ - Desktop Tool
- http://www.liblas.org/utilities/txt2las.html - Convert into binary LAS format.
Sources of Lidar data
- http://environment.data.gov.uk/ds/survey#/download (catalogue of data here: https://github.com/cgutteridge/uklidar )
- http://kitwallace.tumblr.com/tagged/terrain -- blog about terrain data.