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Altitudes & site grid ref precision

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Altitudes & site grid ref precision

by AndyAmphlett » Fri Mar 07, 2014 10:30 am

Hi,

I thought that there might be a problem with calculation of minimum altitude as returned from a query using 'subdivide by altitude (min/max)'. It may relate to how the DDb uses site centroid grid refs, and would like to have this clarified. An example is this query:

Image

I assumed that the min and max values returned by the query are the smallest min or greatest max altitude found within grid squares where the taxon is recorded, as defined by the site grid reference. So in this case, the max altitude for Carex pallescens of 1211m is the max altitude in tetrad NH90V, where the species has been recorded (no doubt the actual location was much lower!). I was struggling to find the record that referred to the minimum altitude. I used two different methods to narrow it down, the more elegant of which was the query below:

Image

But, when you click on the site grid ref for this record (NJ481640), the minimum altitude is shown as 231m, not 105m.. I was about to report this as an error, but noted that the site name indicated that it is a site centroid grid ref. Using the DDb to check altitudes in the monad NJ4864 reveals a min altitude of 105m, just as the initial query returned :). So is the DDb detecting that the site grid ref is a centroid and blurring the precision to monad scale and calculating altitude from that?

NB if I download the record and tick the option to estimate altitude it gives the mean altitude for the 100m grid square (251m). It seems that altitude is being interpreted or calculated differently in these queries. When downloading records and ticking the option to estimate altitude, it would be good if min and max altitude for grid squares could be included in the output, in addition to mean. Would that be possible?

Thanks,

Andy.

(Post title edited to reflect feedback from Tom H.)
Last edited by AndyAmphlett on Fri Mar 07, 2014 6:48 pm, edited 1 time in total.
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Re: Altitudes & site centroids

by admin » Fri Mar 07, 2014 10:49 am

Hi Andy,

Thanks for investigating this so thoroughly.

You are right that the grouped altitude query is using a different calculation (at best only looking at monad square resolution). This was done for reasons of efficiency (the precise altitude values are not currently stored in the database - but are calculated by interpolating from values in a look-up table every time a precise altitude is needed, whereas altitudes for all monads are pre-cached separately so are available to queries quickly). A solution would be to cache the maximally precise altitude estimate in the record table - which is definitely something that could be done and may well be worthwhile.

I've recent been sent a revised set of altitude data for Britain - but need to look in detail at whether it's an improvement of the current model. Using the new set would change how the DDb did altitude estimate calculations - so I need to decide whether to put more work into refining the current system or to switch to the new data.



It shouldn't be difficult to add the min/max altitude columns to exported search results - I'll look into adding those columns.

regards,

Tom
Tom Humphrey
Database Officer, Botanical Society of Britain and Ireland (BSBI)
c/o Centre for Ecology and Hydrology,Maclean Building, Crowmarsh Gifford, Wallingford, Oxon, OX10 8BB, UK.
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Re: Altitudes & site centroids

by AndyAmphlett » Fri Mar 07, 2014 11:03 am

Tom,
Thanks. So just to clarify, my musings that this was something to do with site centroid grid refs was incorrect. The query was making using of pre-cached monad precision altitude information, rather than calculating altitude for the actual 6-figure grid ref?

Andy.
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Re: Altitudes & site centroids

by admin » Fri Mar 07, 2014 11:34 am

Yes, the grouped altitude query is at best only using monad resolution, rather than 6-figure.
Tom Humphrey
Database Officer, Botanical Society of Britain and Ireland (BSBI)
c/o Centre for Ecology and Hydrology,Maclean Building, Crowmarsh Gifford, Wallingford, Oxon, OX10 8BB, UK.
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