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--- metapost [2019/06/03 01:04] – [Symbol Sizing and Positioning] explain align in scrap versus output brucemutton
+++ metapost [2019/06/04 00:02] – [Magnetic effects] tarquinwj
@@ Line 366: / Line 366: @@
   text en "point u:warning" "warning/danger"
+===Magnetic effects===
+Certain rocks can cause a compass to give the wrong reading. This icon can be used to show areas where this happens (ie. where the survey may be unreliable as a result); a spinning compass:
+<code>
+code metapost
+  % a spinning compass
+  def p_u_magnetism (expr P,R,S,A)=
+    scale:=0.5u;
+    halfline:=(0.5u/20S); %half thickness of PenC - pen thicknesses do not scale with S
+    pointheight:=scale*.9;
+    pointwidth:=scale*.4;
+    U:=(scale,scale);
+    T:=identity aligned A rotated (R-20) scaled S shifted P;
+    % a circle
+    thdraw fullcircle scaled 2scale withpen PenC withcolor black;
+    % filled triangle
+    thfill (0,pointheight)--(pointwidth,0)--(-pointwidth,0)--cycle withcolor black;
+    % black triangle outline
+    thdraw (0,-pointheight+halfline)--(pointwidth-halfline,0) withpen PenC withcolor black;
+    thdraw (0,-pointheight+halfline)--(-pointwidth+halfline,0) withpen PenC withcolor black;
+    % spin arcs, a full circle is path 0-8, anticlockwise, starting from the right
+    thdraw subpath (2.4,3.5) of fullcircle scaled 1.5scale withpen PenC withcolor black;
+    thdraw subpath (6.4,7.5) of fullcircle scaled 1.5scale withpen PenC withcolor black;
+  enddef;
+  initsymbol("p_u_magnetism");
+endcode
+</code>
+{{ ::magnetism.png?nolink |}}
+Select this as point type "u" with "-subtype magnetism" in its options. Use it with this line in your layout:
+  text en "point u:magnetism" "magnetism"
 ====Line Symbols====
 ===View whole centerline for underground===