Archive for February 2017

St Mary’s Loch & Loch of the Lowes – images and bathymetry   Leave a comment


Loch of the Lowes on a calm February day. Image courtesy of Dan McGarvie. 


On St Mary’s Loch on a cold but calm December day.


This blog shows the fun I’ve had wandering across these two lochs in the Scottish Borders in an inflatable kayak using a portable sonar. So far, I have completed the underwater contours (isobaths for pedants) on the smaller Loch of the Lowes, so I will provide this map first – in its entirely and also in sections that provide more detail. I shall leave the angler to use these maps as they see fit when deciding on suitable places to fish.


Panorama of Loch of the Lowes. January. Image courtesy of Dan McGarvie.


But first some images, because being out in a kayak on the lochs can be a rather special experience, with the interesting perspective one gets from being on a water-dominated environment. The rare days of calm conditions are especially enjoyable, as the reflections of the surrounding hills are mighty fine, to say the least. And when the only disturbance on the water comes from the ripples generated by the moving kayak, which leaves trails of foam defining the wake. The coldest I’ve been out was -5 Centigrade, but as it was calm the cold was not uncomfortable, and one could watch the splashes freeze on the paddle and on the kayak deck. The captions say a little more.


St Mary’s Loch in more typical conditions. December.


Just before sunrise at St Mary’s Loch. Temperature -5 Centigrade. December.


Stretching the legs, east bank of St Mary’s Loch. Dceember.


The Maps

Technical comments on the sonar and the maps are at the end, but here’s what’s useful to know when viewing (and interpreting) the maps:

  • The underwater contours are at 3 foot intervals. The sonar provides readings at greater resolution than this (e.g. 9.4 feet).
  • The sonar is less accurate at depths less than 4 feet.
  • The colours range from shallow (dark orange) to deep (blue).
  • The colour-depth range of interest to many Pike anglers using deadbait is c.15-30 feet, which is yellow (15 feet) through to pale green (30 feet) on the chart below, which shows depth-colour isobaths down to 60 feet.


Loch of the Lowes new underwater (bathymetric) map

For interest, the original bathymetric map from the 1897-1909 Scottish survey is shown first, followed by the new map. It’s interesting to compare them, and one has to be impressed at the accuracy of the original surveyors who used weighted lines to determine water depths. The maximum depths recorded on both the old and new surveys match up beautifully.


Loch of the Lowes original bathymetric map produced during the 1897-1909 survey.


New bathymetric map of Loch of the Lowes.

The new map (above) and the following more detailed sections are basically screenshots from the app used with the sonar. When I have time I’ll try and learn how to extract the SQL files from the smartphone and import them into a GIS system  such as QGIS to produce larger and better maps. Though this may have to wait till I retire in 4-6 years, when it will become a proper ‘project’. Images that cover the entire Loch of the Lowes in more detail follow, with a water depth indication on each: captions are not provided.













The Sonar and technical stuff

The sonar is a lightweight ball that houses the sonar, a sensor determining water temperature, and a wireless transmitter. The sonar sends pulses of data via wireless to the smartphone, on which live data is shown on a screen – water depth, type of bottom (weedy or rocky etc), and there’s also an option to switch on ‘fish icons’ which will show you individual fish and their depths, as well as a separate icon for shoals of fish.

On your smartphone you download the app and also a copy of the map of the country your body water is in. Your phone locates where you are using its built-in GPS.

Put simply, your smartphone screen shows where you are, and the wireless transmitter from the sonar shows the water depth and temperature of your position. The neat bit is that the track you take across the water is mapped in front of your eyes as you paddle along. By joining up a number of tracks you create the map.


Example of compiling a map by connecting together different paths. Northern end of St Mary’s Loch.

The potential attraction of this to anglers is that a map can be ‘shared’ provided they have a smartphone and load the app. You phone’s GPS will locate where you are on the map, and so (for example) you can stand on the bank and know that 120 feet out there’s a drop-off from 20 to 40 feet. This is because you can tap the map on your phone and at the point at which you tap the water depth will be given. I’ll write a separate blog entry about how to do this and how to access the map data I’ve produced.

For now I’ll leave you with a few images of the kayak and sonar in action.


The sonar is attached to the end of (yes) a landing net pole so that the sonar is in reasonably calm water and does not impede paddling. 



My bespoke sonar housing is cheaper and better than the ‘official’ sonar arm one can buy. It’s basically a frisbee with a hole cut into it, and with some foam pipe lagging for buoyancy. And yes, I occasionally stop and have a few casts.


I find that even in the depths of winter Pike can be tempted to attack lures (I use various small softbait jigs). Unhooking from a kayak is straightforward as one is so close to the water, with the added advantage (to the Pike) that a proportion of the Pike’s body can stay in the water (for support), except when taking a pic as above. I never weigh them, as I like to return them swiftly because they don’t have much energy in winter. But if it was a really big one…?


Top end of Loch of the Lowes. The statue of James Hogg can be seen left of centre.






Posted February 20, 2017 by davemcgarvie in Uncategorized