Sunday, 16 March 2014

Basic Levelling Survey - Comparison Spreadsheet


Between January 1998 – January 2009 I surveyed over 420 hills in Wales using a staff with a fixed spirit level to sight from one point to another.  This procedure was repeated from bwlch to summit until the hill’s drop value was ascertained.  The best description for such a method is a Basic Levelling Survey (BLS).  For an introduction to this surveying method please click {here}.

These basic levelling surveys promoted a number of hills including four to the Nuttalls list, 18 to the Deweys list and 24 to the list now known as Y Pedwarau.

Since September 2008 G&J Surveys, and from December 2013 Mapping Mountains, have had access to survey grade GPS / GNSS equipment that can produce accurate absolute height measurements.

Since September 2008 a number of hills that were promoted to the lists mentioned above through BLS have had their drop values accurately surveyed, either by the Leica 530, Leica GS15, Timble GeoXH 6000 or by level and staff.  To date (March 2014) this has resulted in two demotions of hills from the Deweys list, both originally promoted due a BLS: Iwerddon (SH 688 482) and Cerrig yr Ieirch (SH 758 425).  Another 13 hills have been confirmed as being a Nuttall, Dewey or Pedwar and a further 13 hills have had an accurate survey comparison to the original BLS.  In all there are 28 hills out of the 420 whose drop was attained by a BLS that have now been surveyed by either level and staff and / or GPS / GNSS receiver, as time progresses this figure will increase.

As there is now a comparison of the survey results between 28 hills a statistical analysis can be made to determine the level of uncertainty applied to the surveys conducted with the staff and fixed spirit level.  It is hoped that this statistical analysis will appear on the Mapping Mountains blog in the near future.  But for now the details of the hills surveyed by the basic levelling technique that have also been accurately surveyed appear in a spreadsheet.

To see the Basic Levelling Survey Comparison spreadsheet click {here}

The spreadsheet consists of the following:

Date Accurately Surveyed:  This is the date when each hill was surveyed either by GPS / GNSS receiver or by line survey.

Date (BLS):  This is the date of each basic levelling survey.

Name:  This is considered the most appropriate name of the hill.  Sometimes the name used does not correspond to current Ordnance Survey map spelling or the name may not appear on any map.  Where no appropriate name has been discovered for the hill from any source, the Pt. (for example; Pt. 688m) notation is used rather than making up a name that has no local or historical evidence of use.

Height (m):  This gives the map height in metres of the hill above Ordnance Datum Newlyn (ODN), often referred to as sea level.  Where a height is quoted to a decimal place it implies that the hill has been surveyed by GPS / GNSS receiver (these heights may not match current Ordnance Survey map heights).  Where a ‘c’ (contour) appears preceding the height it means there is no known spot height available.

1:50,000 Map: This column gives the number of the 1:50,000 Ordnance Survey Landranger map that the point surveyed appears on.

1:25,000 Map:    This column gives the number of the 1:25,000 Ordnance Survey Explorer map that the point surveyed appears on.

Grid Reference:  This is the six figure grid reference produced by the GPS / GNSS receiver for the point surveyed or for those hills that have only been accurately surveyed by a level and staff this will be from a hand-held GPS unit.

Drop (Basic Levelling Survey) (ft):  This column details the drop in imperial measurement attained from the BLS.  This value is the relative height of the hill; this is commonly referred to as ‘drop’, ‘prominence’ or ‘reascent’.  The drop is the height difference between the summit and bwlch connecting the hill to next higher ground along the watershed. 

Drop (Basic Levelling Survey) (m):  This column details the drop in metric measurement attained by the BLS.

Drop (spot heights on OS maps) (m):  This column details the hill’s drop if a spot height for both summit and bwlch appear on an Ordnance Survey map.  The appearance of a spot height does not mean that it is placed at the uppermost part of the hill or at the critical bwlch. 

Drop (Line Survey) (m):  This column details the drop value in metric attained from a line survey (if one has taken place).  This form of survey gives the most accurate method to determine the height gain from bwlch to summit. 

Drop (GPS / GNSS Receiver) (m):  This column details the drop value in metric attained from a survey conducted by the Leica 530, Leica GS15 or the Trimble GeoXH 6000 (if one has taken place).  

Leica Disto (m):  The Leica Disto A8 is capable of measuring distance and angle.  To date it has only been used on one hill; Castell y Gwynt (SH 654 581) that has also been surveyed by a BLS.  

Distance from Bwlch to Summit (m):  As the BLS is easier to conduct over shorter horizontal distances (less margin for error in vertical alignment), the distance between the bwlch and summit is important to document.  Distances quoted are approximate.

Status (surveyed for):  This gives the class of hill applied to each survey.  For now the details in the spreadsheet only apply to the class of Nuttall, Dewey and Pedwar. 

Comments:  The last column is used to document if the hill has had more than one BLS, or if the hill appears in the relevant list by another name when compared to the one deemed the most appropriate by the blog author and details of the GPS / GNSS receiver used for the survey is also given in this column.

To see the Basic Levelling Survey Comparison spreadsheet click {here}

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