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I have two questions :

  1. How does tile creation in geotiff improve performance and how will reading of tiled gtiff images be done?

  2. Adding an overview to a geotiff image means we are resampling the original image to a small size and storing it with the original image so that when we are trying to read a large image, the overview will be read first and then the original image (so that reading latency is hidden).

Am I right on this? What is the point of adding more than one overview?

Can somebody explain briefly about the geotiff format?

Thanks

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1 Answer 1

Some clarifications in your question are needed. On what technology or platform are you creating and serving these images? Various tiling schemes, pyramids and overviews exist but they have little to do with the format of the images.

For example in ArcGIS, raster pyramids take the form of a .ovr file for single raster datasets. For mosaic datasets, overview images can be created either in lieu of or in addition to pyramids. Lastly, in a cached map service, tiles are created at a series of scales for optimal retrieval and transmission properties.

These are all independent of the TIFF format itself, whose specification does not define any of these methods. The GeoTIFF format (see specification) is an extension of the TIFF format that simply adds georeferencing information into the file header and as such has no bearing on this discussion.

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As in gdal we can set TILED=YES(By default stripped TIFF files are created. This option can be used to force creation of tiled TIFF files.) How these tiles will improve performance and how overview addition (using gdal) helps performance. –  user8119 Jun 11 '12 at 1:36
4  
This is explained in the TIFF specification, section 15, "Tiled Images": "For low-resolution to medium-resolution images, the standard TIFF method of breaking the image into strips is adequate. However high-resolution images can be accessed more efficiently — and compression tends to work better — if the image is broken into roughly square tiles instead of horizontally-wide but vertically narrow strips." Note this is talking about compression within a single image, not a series of images like a cached map service. –  blah238 Jun 11 '12 at 1:42

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