Vertical and clearance heights
Measurement of Vertical and Clearance Heights
We can see from the Title Information Block (Fig 2.10) that vertical heights above sea level are also measured in metres. Remember, ‘drying heights’ are measured from Chart Datum (CD) and give an indication of how much of the seabed / intertidal zone will be above sea level around Lowest Astronomical Tide (LAT) when the tide goes fully out. Vertical Heights, on the other hand, are measurements of features above the high tide level including lighthouses, mountains, bridges etc. We can divide the vertical heights of charted objects into two general groups:
Vertical Heights – Charted objects or features that we do not want to go under, like lighthouses, mountain tops, church spires etc. Fig 2.11 ‘Arrow A’ shows a spot height of 353 metres above sea level.
Clearances – Charted objects we want to go under, like bridges, pylons, overhead cables etc. Fig 2.11 ‘Arrow B’ shows a clearance of 14m under the bridge.
If we want to measure the height of a mountain or lighthouse we would measure this from Mean High Water Springs. However, on some rare occasions the water may extend a little higher, and if we wanted to get under a bridge or electricity pylon stretching across a river, it would be better for us to measure this clearance height from the Highest Astronomical Tide (HAT) level. Only clearances, where we have to get underneath some structure are measured from Highest Astronomical Tide, everything else, from mountain tops to church spires are measured from Mean High Water Springs datum. We look more closely at these different reference points for measuring heights in another module but it is useful for us to be able to start thinking about the different datums we measure heights from:
Lowest Astronomical Tide (LAT)
Chart Datum (CD)
Mean High Water Springs (MHWS)
Highest Astronomical Tide (HAT)
As we have seen clearance heights are measured from Highest Astronomical Tide (HAT) level, which is the highest the tide will ever reach. The charted height of the bridge (Fig 2.12) is a measurement of the distance from underneath the bridge to the Highest Astronomical Tide level. This HAT sea level is the highest tide we can expect, so the clearance measurement is the ‘least’ clearance we can expect under the bridge. In fact we would probably always expect more clearance as it is rare for the tide level to get this high and tide level is usually always lower than HAT.
Fig 2.12 shows a yellow-buff colored bride extending over, to Hinder Island. The clearance is highlighted and measured as 4.5 metres. This number is bounded above and below with little lines that symbolizes clearances. We have 4.5 metres clearance below the bottom of the bridge and the Highest Astronomical Tide Level (HAT). So even under the most extreme astronomical circumstances when the tide is fully in there will always be 4.5 metres of clearance under the bridge and at any other time we would expect more. In other words, the charted clearance height gives us a ‘worse case scenario‘ clearance under the bridge.
When can we get under the bridge?
If you look at RYA Training Chart 4(F) you will see that the bridge spanning the entrance on the approach towards Blackmill Marina has a clearance of 15 metres (Fig 2.13). If we are in control of a barge that has a crane height or air draft of 16.5 metres above the waterline, we have a little problem, as we see form the chart that the clearance of the bridge is only 15 metres!
So we know that at Highest Astronomical Tide the crane barge will not fit under the bridge (Fig 2.14). We will need to calculate when the tide is at least 1.5m below the level of HAT to squeeze the crane under the bridge. It is always prudent to have at least 1 metre safety margin between the top of the crane and the bottom of the bridge. This means that we must calculate when the tide is at least 2.5 meters below the level of Highest Astronomical tide before we can safely get the crane under the bridge. We will be examining this type of tidal calculation in a future module where we need to understand how to use tide tables.
The height of a lighthouse, a charted object.
If you examine the charts and look for summits of hills, churches, radio masts, lighthouses you will see that there is a measurement of the height of the charted object. The information for a major lighthouse at Point Victoria (top left of RYA Training Chart 3) describes the characteristics of the lighthouse, Fl.10s72m23M (Fig 2.15). This is important information and will be looking more closely at light characteristics in Module 3. The information also includes the height of the lighthouse above Mean High Water Springs. The lighthouse is 72m above sea level. This does not mean that the lighthouse structure is 72 metres high, just that the centre of the light is 72 metres above Mean High Water Springs (Fig 2.16).