Comparing UK Ordnance Survey And SRTM Elevation Data

Introduction

This page compares three free sets of UK terrain elevation data:

  1. OS Landform Panorama (LP)  this is a legacy OS product which is no longer being updated, and has been superceded by the next choice Terrain 50.  See also …
       Using Ordnance Survey Landform Panorama Elevation Data
  2. OS Terrain 50 (T50) this is OS's current free product which supercedes the above.  See also …
       Using Ordnance Survey Terrain 50 Elevation Data
  3. SRTM 3 Arc Second this is a free NASA product.  See also …
       Using Shuttle Radar Topography Mission Elevation Data

Comparison

The pages linked above describe how to obtain and use the individual products.  The purpose of this page is to note some of the major pros and cons of each product, and to compare them numerically.  To begin:

This lack of complete SRTM coverage of The Shetlands can be overcome by remaking the relevant tiles from OS data.  However, this raises questions of which OS data set to use and what procedure to follow in conversion.  In particular:

This author decided to resolve these questions by trialing all the possibilities and choosing the combination that resulted in data that was least different from the original SRTM, where the latter existed.  It should be noted that this is not necessarily the same thing as choosing the 'best' method, in the sense of giving the most accurate representation of actual UK terrain, but in the absence of independent absolute data against which to measure accuracy, it's probably the next best thing.  Given this target, it is obvious enough that when using T50 data using a mask to exclude the sea will give better agreement, because fewer points will be changed, and if doing so for T50, it needs to be done for LP as well, so that the comparison between the two is like for like.  The results of the trials of other possibilities are listed below.  Although no one trial is best by every criterion, Terrain 50 without any of the possible adjustments (bold) gives the best overall agreement with SRTM.  A surprise is that it seems to be better not to attempt to correct for Geoid height, whereas to do so would seem to be the logically correct thing to do.

  Tiles CorrectedPoints CorrectedData Voids FoundData Voids FilledMinimum CorrectionMaximum CorrectionAverage Correction
Landform Panorama-L7138,989,6477,3757,37514473.745
-L -A7139,570,8327,3827,38214474.019
-L -G7139,268,6407,3757,37514473.804
-L -A -G7139,791,8227,3827,38214464.073
Terrain 50 7135,912,1717,3757,37514893.268
-A7141,175,5337,3827,38214907.039
-G7136,458,0957,3757,37514893.318
-A -G7141,344,4917,3827,38214907.064
Command-Line Switch Meanings
-AUse alternative cell centring as defined above
-GCorrect for EGM96 geoid height
-LUse Landform Panorama heights

Although in the numerical comparisons above the amount of maximum disagreement may seem alarming, with the exception of the point of maximum correction discussed below, these are mostly in The Shetlands where SRTM data is either non-existent or of low quality.  Further, it should be remembered that while indeed there are a significant number of significant differences, all of which would require explanation in an ideal world, mostly there are a large number of comparatively small differences.  This can be demonstrated by visual comparison between the original SRTM tiles and those made as per the best result above, but images for the entire UK would be too large to display here (but are included in the OS2SRTM-All.zip below), so instead only a test section across the approximate centre of the UK mainland at 55°N is examined here.

The first image is a simple comparison where colour intensity is proportional to differences in height between the sources; the second is graded so as to emphasise pixels where the sources disagree by more than 10m (half intensity) and 100m (full intensity); the third shows colour at full intensity wherever the sources disagree by 1m or more.  While the last image is heavily coloured, showing that disagreement is widespread, the first is predominantly greyscale, showing that this disagreement is mostly small in magnitude.  The middle image shows that nevertheless there are enough significant disagreements to merit caution in the use of both sets of data. 

Note:  In all cases greyscale intensity is roughly proportional to terrain height, while colour intensity is determined by differences in height between the sources, as described above, with areas where SRTM is higher tending to blue and T50 higher tending to green, except for data voids in the original SRTM, which are shown in magenta.

Greyscale comparison between SRTM and OS Terrain 50 across the UK at 55°N Graded comparison between SRTM and OS Terrain 50 across the UK at 55°N Full comparison between SRTM and OS Terrain 50 across the UK at 55°N

Although such comparison images for the entire UK cannot be displayed here, when examined they throw up enough of interest to merit further discussion, to understand the significance of which it is necessary to know that SRTM radar data was collected by the shuttle orbiting roughly NW-SE and NE-SW, while OS data is combined from land surveys and aerial stereoscopic photographic imaging.

Shetland Conversion

The programs that made the above comparisons will also remake just the corrupted / missing SRTM tiles covering The Shetlands, by giving them the -S command-line switch.  If SRTM1 rather than SRTM3 tiles are required, also give the -1 switch, though this has been less well tested.  Ready-made SRTM3 Shetland tiles are available via the link SRTM3-OST50-Shetlands.zip below.

Downloads

For those that wish to explore this subject further, these downloads may prove useful:

Note:  No breach of copyright of source data from SRTM or OS is intended, such copyright remains with the original sources.  Additional material created by Charles Macfarlane is available under the Creative Commons terms relating to my site.