Telecommunications

This exercise is a continuation from exercise 1 and requires the data from exercise 1 to be downloaded.  Please also download Exercise 2 data attached.

This exercise contains OS data © Crown copyright and database right 2016.  OS OpenData is free to use under the Open Government Licence (OGL).  OS Terrain 50 user guide and technical specifications can be found here.  The data itself can be downloaded as an attachment below.

Objective: Open a large MRR grid file and clip to a country boundary to improve the visualisation and remove erroneous data around the coast.

• Navigate to Home > Open Table and select GB OST50 UK.tab from Exercise1 download folder. 
• Navigate to Home > Open Table and select UK Boundary.tab from Exercise 2 download folder.

*Note both maps do not need to be visible in a map window or in the same window for the clipping function to work.

1. Navigate to Raster > Raster Operations > Clip. *Ensure that the settings in Clip details pane on the right hand side of the MapInfo interface are set to Retain Inside and Region(Polygon). Polygon will be set to UK Boundary.
2. In Output File option, select the file explorer button and save the file as GB OST50 UK CLIP. Click Process. (This may take a few minutes).
3. Close the original grid file GB OST50 UK and leave GB OST50 UK CLIP open. This file will be used in exercise 3.

As you can see, this is very quick and easy process.  Below is an explanation of the compression methods supported for the new clipped file:

LZ4 Compression - It is a lossless compression method that can be applied to any kind of field. The user does not need to specify a compression level. Its primary advantage over other lossless compression methods is the very high speed of encoding and decoding and the low memory and computing requirements. However, it will typically achieve lower compression ratios than other lossless methods. Because of its superior efficiency, LZ4 is widely used by operating systems and file systems.

ZIP Compression - It is a lossless compression method that can be applied to any kind of field. By specifying a compression level of 1 to 9 the user can tune the method to provide higher performance (and lower compression ratios), or lower performance (and higher compression ratios). It achieves higher compression ratios than LZ4 but this is always at the expense of encoding and decoding speed and memory consumption. This industry standard method is widely used in file compression tools.

LZMA Compression - It is a lossless compression method that can be applied to any kind of field. By specifying a compression level of 1 to 9 the user can tune the method to provide higher performance (and lower compression ratios), or lower performance (and higher compression ratios). This method can produce higher compression ratios than LZ4 and Zip, but at considerable expense. For compression levels of 1 to 4, the method is generally fast and cheap but may not provide significantly better compression than Zip. At compression levels 5 to 9 the method significantly improves compression ratios but the cost is large in terms of time, memory consumption and computing requirements. This industry standard method is widely used in file compression tools.

PNG Compression - It is a lossless compression method that can be applied to 'Image' fields only. The user does not need to specify a compression level. It will achieve higher compression ratios than the general lossless compression methods (LZ4, Zip, LZMA), but the encoding performance is relatively poor. It is an industry standard image compression method commonly used for internet imagery and image editing software.

JPEG Compression - It is a lossy compression method that can be applied to 'Image' fields only. By specifying a compression level of 0 to 9 the user can control the quality of the image from highest quality (largest size) to lowest quality (smallest size). In general, it will achieve higher compression ratios than any of the lossless compression methods and the encoding and decoding performance is very good. It can only be used for single component or three component color (R, G, B, RGB or BGR). If an alpha channel is present this will be written separately using a lossless encoder. It is an industry standard image compression method commonly used for internet imagery and image editing software.
Note: For 'Continuous', 'Classified' and 'Image Palette' fields you must select a general lossless method. For 'Image' fields you can select either lossless or lossy image compression method.

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Chris Jenkins
Pitney Bowes Software Ltd
Henley-on-Thames
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