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First; I can see a band as synonymous with a channel in an image editing program like GIMP, is this correct?

If so then bands play the role of color mixing and masking. Are there any other functions of a band in GIS ie. would I ever find more than four bands in a layer?

Second; dimension applies to a raster size in the project CRS, right? This is pixel independent (the graphic card sorts it out). Change the CRS and raster dimensions change. Change resolution and the raster dimensions change.

Third; anything immediately apparent in the histogram below as to the cause of binary resolution (black hole and blank space) and what parameters to alter? Thanks so much for the help.

histogram fail

I just noticed from this post :

QGIS rounds up the coordinates to six decimals when saving layer changes

that numbers are rounded to six decimal places. This is a sample of my coordinate data:

-75.10978995 40.02942646

-75.0801132 40.06566807

-75.0801132 40.06566807

-75.03800228 40.03786083

-75.24746918 39.95963876

-75.23858404 39.96544434

-75.23560721 39.94817612

-75.22978442 39.98298566

-75.20350911 39.96409881

-75.17341205 39.99858338

Is it possible that I should use the raster calculator to make these numbers bigger?

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That rounding is with vector stuff not raster. – Nathan W Feb 19 at 12:40
Isn't the point data, vector data? Is the heat plugin taking vector parameters or just analyzing a layer of rasters? – Fred Feb 19 at 12:49
If so, would weight constitute another band in the raster layer? Which begs the next question; if so, can we have multiple weighting bands and thus more than four bands in a raster layer? – Fred Feb 19 at 12:56

1 Answer

up vote 1 down vote accepted

BANDS

When working with e.g. scanned maps raster the three (four) bands are exactly the same as RGB(A) channels in e.g. GIMP. But that does not apply to other kinds of raster data.

For example, remote sensing data such as multispectral images acquired from satellites can contain (almost) any number of bands with each pixel value for a specific band corresponding to the intensity of electromagnetic radiation at a specific wavelength. With multispectral images thinking simply in terms of "color mixing and masking" is not going to be helpful: the interpretation of the bands depends on how the phenomema being researched reflect electromagnetic radiation at different wavelengts.

Or you could have a single band raster, where each pixel value corresponds to some measured or interpolated variable, such as rainfall.

DIMENSIONS

At least in QGIS the dimension of a raster is about the size of the image in pixels, i.e. its Y * X size independent of the size/and unit of one pixel. So the dimension does not change when you change the CRS, thought the measured size of the area covered by the raster might change depending on the projection.

If you change the resolution of the raster (i.e. resample it) the dimension changes, but the measured size of the raster will not change as long as the new pizel size does not leave a remainder (i.e. dimensions divided by pixel size gives a whole number).

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Thanks, this is helpful. Just to clarify; you meant: 'dimension of a raster (layer) is about the size of the image in pixels...' right? The dimension of a raster is the size of a unit in the raster layer, Yes? – Fred Feb 19 at 14:04
Yes, the dimension is the size of the raster layer in pixels. And measured size would be e.g. the size in meters or whatever the units of the CRS are. That depends of course on the size of one pixel in the data set. I'm not quite sure what you mean by, "The dimension of a raster is the size of a unit in the raster layer", though. – Torsti Feb 20 at 7:38
Am I thinking clearly? A raster layer is an array of rasters, the smallest unit being one raster (interpreted differently for different scales and monitors as an array of pixels). – Fred Feb 20 at 15:39
Should I not think of it this way for this application? : "A Raster defines values for pixels occupying a particular rectangular area of the plane, not necessarily including (0, 0). The rectangle, known as the Raster's bounding rectangle and available by means of the getBounds method, is defined by minX, minY, width, and height values. The minX and minY values define the coordinate of the upper left corner of the Raster." docs.oracle.com/javase/6/docs/api/java/awt/image/Raster.html – Fred Feb 20 at 15:45
I'm not sure I understand... In a GIS context a raster is a two (or three) dimensional array where the array index designates a geometric position. If you mean by a raster layer being an array of rasters, that a raster layer can consist of multiple bands then I guess you are correct, but I've never heard a multi-band raster called an array of rasters. You don't usually say that a RGBA image is an array of four channels, though that might well be a correct interpretation. In GIS, rasters consist of cells, which is a different term for pixel, and 3d cells are called voxels (volume pixel?). – Torsti Feb 20 at 17:50

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