That ROCK in the entrance to HA'THAYIM (VON DONOP) MARINE PARK... Detailed info (UPDATED!)

Depths (in feet, uncorrected for tide)

UPDATE May 17, 2021: CHS "nonnavigational" soundings ("NONNA10") indicate the depth over "the rock" is 0.6m at zero tide. ALSO, the rock immediately north has a least depth of 1.8m. This too would be an issue for boats with a 6' draft (e.g., Pelagia). Text below has been revised.

**Take home: Subtract 0.5m (1.64') from soundings to be safe(r)**

Updated March 14, 2021:

Von Donop Inlet (properly known as Ha'thayim Marine Park) is a favourite anchorage of ours in the Desolation Sound/Discovery Islands area, situated at the northern end of Cortes Island.

As every decent cruising guidebook notes, the narrow entrance has a rock approximately mid channel; the exact depth of this "rock" is not well documented (the official chart shows a "+", indicating a rock "with less than 6 feet over it a zero tide"). Usual navigation instructions are to "hug the western shore". How close to this shore, however, is an issue, as at some tide levels, depths are also too shallow if one hugs the shore a little too closely!

Every year, sailboats hit this rock. Fortunately, we have not, but we know some who have. 

The official CHS chart is not very highly detailed (commercial charts, such as Navionics, derive their data from the CHS chart). We do not depend on the chart for the rock's location, as we have found the "+" symbol to be only approximately correct. Instead, in the past we hugged the western shore when we were passing the "second yellow bare patch on the eastern shore". It always worked for us, but was never a very satisfactory method as (i) hugging the western shore had depths going very low, and (ii) it doesn't really let us know where the rock is located.

This year, David used his new Deeper Pro+ portable sonar to do a detailed scan of the entrance channel (at higher tide) in order to finally figure out this rock.

What we learned:

• the "rock" is actually a pretty wide rock, least depth of about 1.0 m (3.28') 0.6m (1.9') at zero tide, covering 2/3rd of the width of the channel. A serious issue for most sailboats, and some powerboats
  
• there is, indeed, a deep (enough) channel to the west of this rock
• there is another rock immediately northwest of the rock with a least depth of about 1.9 m (~6.2':  1.85m (~6.06'): so only a worry for boats with keels 6 over feet)
  
• further north, there is a very shallow area on the west shore jutting out a little, north of these rocks. Hugging the west shore therefore not a good idea at the northern entrance to the narrow section 
• do not consider going to the east of the rock (i.e., don't go along the eastern shore)
  

 

The following figures provide more detailed information:

 

Uncorrected depths in ft: the arrow points to place of 1m 0.6m corrected depth (sorry, not "3.8ft")

 

The Deeper Pro+ sonar app creates a bathymetric map as you scan. The above map was created from many passes up/down/across the narrow section. These passes did not include all of the area within the map (see figures below); the Deeper app interpolates/extrapolates from the data to "fill in" the map. (Thus, if I missed a shallow area, the map too may miss this.) I thus tried to concentrate my scans around the rock/shallow areas to improve accuracy.

 

Unfortunately, the Deeper app does not provide for a correction for tide level -- seems it only understands "lakes" -- so the depths in this map reflect the higher tide level at which I scanned; they must be corrected for tide level (thus, the "8.2 feet" above is actually <4 feet at low tide).

Some extra detail (sorry... ): If one uploads the Deeper Pro+ scan data to the Deeper cloud on the internet (each Deeper owner gets an account), one can login into their account on the Deeper Lakebook page (maps.fishdeeper.com) and view more detailed data, including the actual depth readings. One can also download all the depth recordings, their latitude/longitude. and time as a CSV text file. Of course, none of these depths are corrected for tide level.

[As much as I like the Deeper system, requiring one to have good internet access in order to get the accurate location information -- i.e., the lat/long -- is a serious flaw and ridiculous. Most of British Columbia's coastline is without internet access, even by cell (assuming one would want to use up all your cell data doing this). I had to wait until we stayed in a marina weeks later to upload then download the detailed data.]

After downloading the detailed data into a CSV textfile, I reviewed the Redonda Bay tide data (obtained using a tide app based on XTIDES; inaccuracies can occur), and used Excel to correct the depths to zero tide ("datum"). I then converted a CSV text file of these data to a GPX file using GPSU Import File Converter and then GPS Utility, both available from Alan Murphy's https://www.gpsu.co.uk/.  Note: these corrections are not official; it is safest to assume they could err by 0.5m.

The depth data included 747 soundings... far too many. I decided to reduce the data to (i) depth less than 2m (6.6 feet) as well as (ii) a "safe" route for most boats (unless your boat draws 8 feet!). These data are plotted on a geo-referenced satellite image of the narrows.

As shown below, I converted image to a kap chartfile (which can be read by most charting software, but not onboard dedicated chartplotters), and saved the <2m waypoints and safe route into a GPX file (which CAN be read by most dedicated chartplotters, after conversion, as well as most charting software).  For those wishing to download, see below.

 

 

Note the "+" on the CHS chart is a little north of the actual rocks. The blue line indicate the "safe" route (for Pelagia, at least). 

UPDATE (March, 2021): Paul Higgins has updated his excellent SAT2CHART program to now input depths (in the form of a XYZ file). Below is screenshot of a MBTILES (better than kap) file:

MBTILES file with subset of depths added (in metres). Safe route in blue. Yellow depths are <2m

Now, I have to go kick myself for providing information that makes it easier to enter Von Donop and perhaps increasing the summer crowds. Just kidding! (Although VonDonop is indeed very crowded in July/August!) 

Hopefully this detailed information make entry/exit safer for all. For added safety, subtract 0.5m from the depths to account for possible errors in tide-level correction.

Nevertheless, even with this detailed information, we still go very slow by the rock... one never knows what's lurking below.

Stay safe!

CLICK HERE (updated Mar 14, 2021) to download from Google Drive, the zip file containing the above information, including:

• Detailed description and instructions, as well as caveats
• geo-referenced satellite image kap file
• geo-referenced satellite image MBTILES file, WITH DEPTHS (in metres)
  
• GPX file containing <2m depth waypoints and "safe" route waypoints

NOTE: Use at your own risk; there are no guarantees these will be safe for you.