{"id":1925,"date":"2018-12-10T09:16:09","date_gmt":"2018-12-10T17:16:09","guid":{"rendered":"http:\/\/tradewindssailing.com\/wordpress\/?p=1925"},"modified":"2020-04-13T06:49:03","modified_gmt":"2020-04-13T13:49:03","slug":"coast-piloting-part-4-lines-of-position","status":"publish","type":"post","link":"https:\/\/www.tradewindssailing.com\/wordpress\/general\/coast-piloting-part-4-lines-of-position\/","title":{"rendered":"Coast Piloting Part 4 – Lines of Position"},"content":{"rendered":"

This tip could have been named “Obtaining a Fix.”\u00a0 Maybe it should have been, because that’s what its all about.<\/p>\n

Dead Reckoning is a great thing.\u00a0 \u00a0It’s a way to figure out approximately where you are.\u00a0 However, the goal of every navigator to to know exactly where they are as often as possible.\u00a0 This is known as a “fix,” and you can’t obtain a fix without first establishing two or more lines of position (generally referred to as LOPs.)\u00a0 \u00a0 There are several ways to establish an LOP, and we will be discussing a number of the more commonly used.\u00a0 Especially those that don’t require specialized equipment not normally found on a small sailboat.\u00a0 A sextant is a great tool, and can be used in a variety of ways to find an LOP, however, I don’t generally have one on board when I’m sailing, and I don’t recall ever having seen a pelorus on a 42 foot sailboat.\u00a0 What we do have are compasses, depth sounders, and radars, so for the most part we will limit the discussion to LOPs\u00a0 generated by them. Need help with your project? This company http:\/\/gravitymarine.com\/<\/a> is the right choice for helping collect the scientific data of any marine project.<\/p>\n

There is one very important thing to keep in mind when talking about obtaining a fix.\u00a0 One LOP will never work to generate a fix.\u00a0 It takes at least two, and better yet, three or more.\u00a0 If everything and everyone was perfect, two bearings to different objects would provide a very precise location.\u00a0 Unfortunately, perfect isn’t going to happen.\u00a0 If you are looking to get bearings to an object, it’s hard to maintain closer than 5 degrees of accuracy using a hand held compass on a rolling deck, and who knows, maybe you are standing too close to something magnetic (like the hand held VHF radio in your PFD pocket).\u00a0 Stuff happens.\u00a0 The more redundancy you have, the less errors are going to impact you.<\/p>\n

Bearing to an object<\/strong> – A compass bearing taken of two or more charted objects generally provides one of the more accurate fixes.\u00a0 Using day 1 of the Tradewinds ACC class as an example, after exiting the Golden Gate, the class proceeds to the buoys marking the Main Ship Channel through the San Francisco Bar.\u00a0 At G “8”, a decision needs to be made.\u00a0 Continue through the buoyed channel, or turn south now.\u00a0 Seas over the bar are 6\u00a0 to 7 feet, with winds out of the NW at 10 to 15, so it is determined a turn south at that location would present no dangers to vessel or crew.\u00a0 \"\"<\/a>The turn is made at 0954 and a DR begun on C 180.\u00a0 The DR projects positions at 1000, 1100, and 1200, with appropriate expansion circles (see part 1 for an explanation).\u00a0 At 1100 a bearing is taken of a charted TV antenna (B 053) and San Pedro Rock (B 153).\u00a0 Each bearing is marked on the chart.\u00a0 The LOPs are highlighted in yellow for visibility.\u00a0 The resulting fix puts the vessel .6 miles (drift) on a bearing of 109 degrees (set) from the 1100 DR position.\u00a0 The DR is reset from the fix.\u00a0 the navigator now has information regarding set and drift, which should be applied to the reset DR.\u00a0 For additional information regarding set and drift, how to plan for it, and how to correct for it, take a look at the Skipper’s Tip entitled Coastal Pilot Part 3 – Course to Steer<\/strong>.<\/p>\n

\"\"<\/a>\"\"<\/a>There are two commonly used hand bearing compasses.\u00a0 The first is much like a pistol grip.\u00a0 There is a front and a rear sight, which are lined up on the object, and the bearing is read from the compass.\u00a0 The second is puck shaped and is held to the eye in use.\u00a0 A prism allows both the object and the compass reading to be in focus at the same time.\u00a0 Both work, however the puck shaped compass is easier to use while the pistol grip compass is less expensive.<\/p>\n

\"\"<\/a>Depth Contours<\/strong> – In Coast Piloting Part 2 – Entering an Unknown Port<\/strong>, depth contours were used to mark danger areas.\u00a0 In Coast Piloting Part 1 – Dead Reckoning<\/strong>, a depth contour was used as a second LOP in conjunction with the bearing to a light in order to obtain a fix.\u00a0 In this illustration, the 10 fathom contour is marked in read as a danger area.\u00a0 You may notice that the 20 fathom contour (marked in yellow) runs nearly straight for nine miles, keeping approximately 2 miles offshore.\u00a0 A vessel returning from Drakes Bay to San Francisco might sail to the 20 fathom contour and follow it south.\u00a0 If the depth indicated on the depth sounder shows an increase, the helm can alter course a few degrees toward shore.\u00a0 If shallower, altering course a few degrees seaward puts the vessel back on track.\u00a0 As you can see, a depth contour marked on a chart can be a valuable tool.\u00a0 However, you must use them with discretion.\u00a0 The depth contours may not be distinct enough to be located.\u00a0 The area south of the San Francisco Bar is quite flat and depth isn’t going to help much in obtaining a fix.\u00a0 Also, depth surveys may be quite old and depending on location, the depths may have changed dramatically.<\/p>\n

Running Fixes<\/strong> – Running fixes are second only to set and drift in creating confusion in the mind of a new navigator.\u00a0 This is due in large part because of the way running fixes are described.\u00a0 There are some pretty confusing statements … “Advance the LOP forward the distance traveled between bearings.”<\/p>\n

\"\"<\/a>\"\"<\/a>As an example.\u00a0 While traveling a Course of 180\u00ba from Drakes Bay bearing is taken of a prominent point.\u00a0 The DR position at that time is marked.\u00a0 For simplicity, I have labeled it “A”.\u00a0 The bearing is marked off and labeled 0945 (the time the bearing was taken).\u00a0 At 1015, a second bearing of the same point is taken, (it doesn’t have to be that way it can be a different object), marked off on the chart, and labeled 1015.\u00a0 The DR at 1015 is plotted.\u00a0 Again, for simplicity, I have labeled this DR position “B”.\u00a0 The distance between DR position A and DR position B is 2.5 miles, therefore, 2.5 miles is measured from the point where bearing 0945 crosses the DR line.\u00a0 A line parallel to “0945” is marked off crossing the DR line 2.5 miles from where\u00a0“0945”\u00a0 crosses.\u00a0 This new line is labeled “0945-1015”.\u00a0 The location where “1045” and\u00a0“0945-1015” cross is your fix.<\/p>\n

Distance Off<\/strong> – the final method of achieving an LOP we will be discussing is distance off.\u00a0 Actually, its not an LOP, it is a circle of position, but works the same even though its drawn as an arc.\u00a0 The easiest method I know of to obtain distance off is by using radar.\u00a0 Do not go off shore in Northern California without radar.\u00a0 There is too great a chance of fog and radar will be the only measure of safety you have available to you to protect from dangers you can’t see.\u00a0 The concentric rings on the radar make it easy to determine distance off one the object has been identified on the radar screen.<\/p>\n

The second method of finding distance off involves something called “angle off the bow”.\u00a0 While on a vessel traveling a course of 180\u00ba, a bearing\u00a0 of 150\u00ba is taken to an object.\u00a0 That object is 30\u00ba off the bow.\u00a0 As the vessel moves, the bearing to the object will decrease, increasing the angle off the bow.\u00a0 When the bearing reads 120\u00ba, the object is 60\u00ba off the bow (double the original reading of 30\u00ba).\u00a0 The distance traveled between the time of the first and second bearings is equal to the distance off from the object.<\/p>\n

We have explored the more commonly used LOPs to result in a fix.\u00a0 Using them will provide you with assurance of exactly where you are, even though you are in unfamiliar waters.\u00a0 All it takes is a few tools, and some practice.<\/p>\n","protected":false},"excerpt":{"rendered":"

This tip could have been named “Obtaining a Fix.”\u00a0 Maybe it should have been, because that’s what its all about. Dead Reckoning is a great thing.\u00a0 \u00a0It’s a way to figure out approximately where you are.\u00a0 However, the goal of … Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":14,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"font":"","enabled":false},"version":2}},"categories":[4],"tags":[],"class_list":["post-1925","post","type-post","status-publish","format-standard","hentry","category-general"],"jetpack_publicize_connections":[],"jetpack_featured_media_url":"","jetpack_likes_enabled":true,"jetpack_sharing_enabled":true,"_links":{"self":[{"href":"https:\/\/www.tradewindssailing.com\/wordpress\/wp-json\/wp\/v2\/posts\/1925","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.tradewindssailing.com\/wordpress\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.tradewindssailing.com\/wordpress\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.tradewindssailing.com\/wordpress\/wp-json\/wp\/v2\/users\/14"}],"replies":[{"embeddable":true,"href":"https:\/\/www.tradewindssailing.com\/wordpress\/wp-json\/wp\/v2\/comments?post=1925"}],"version-history":[{"count":15,"href":"https:\/\/www.tradewindssailing.com\/wordpress\/wp-json\/wp\/v2\/posts\/1925\/revisions"}],"predecessor-version":[{"id":2465,"href":"https:\/\/www.tradewindssailing.com\/wordpress\/wp-json\/wp\/v2\/posts\/1925\/revisions\/2465"}],"wp:attachment":[{"href":"https:\/\/www.tradewindssailing.com\/wordpress\/wp-json\/wp\/v2\/media?parent=1925"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.tradewindssailing.com\/wordpress\/wp-json\/wp\/v2\/categories?post=1925"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.tradewindssailing.com\/wordpress\/wp-json\/wp\/v2\/tags?post=1925"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}