diff --git a/doc/layers.png b/doc/layers.png
index 2f7c3757..d67c7a0f 100644
Binary files a/doc/layers.png and b/doc/layers.png differ
diff --git a/doc/layers.svg b/doc/layers.svg
index 7d736e12..b9191a4f 100644
--- a/doc/layers.svg
+++ b/doc/layers.svg
@@ -17,6 +17,32 @@
    sodipodi:docname="layers.svg">
   <defs
      id="defs4">
+    <marker
+       inkscape:stockid="Arrow1Lend"
+       orient="auto"
+       refY="0.0"
+       refX="0.0"
+       id="Arrow1Lend"
+       style="overflow:visible;">
+      <path
+         id="path3828"
+         d="M 0.0,0.0 L 5.0,-5.0 L -12.5,0.0 L 5.0,5.0 L 0.0,0.0 z "
+         style="fill-rule:evenodd;stroke:#000000;stroke-width:1.0pt;marker-start:none;"
+         transform="scale(0.8) rotate(180) translate(12.5,0)" />
+    </marker>
+    <marker
+       inkscape:stockid="Arrow1Mend"
+       orient="auto"
+       refY="0.0"
+       refX="0.0"
+       id="Arrow1Mend"
+       style="overflow:visible;">
+      <path
+         id="path3834"
+         d="M 0.0,0.0 L 5.0,-5.0 L -12.5,0.0 L 5.0,5.0 L 0.0,0.0 z "
+         style="fill-rule:evenodd;stroke:#000000;stroke-width:1.0pt;marker-start:none;"
+         transform="scale(0.4) rotate(180) translate(10,0)" />
+    </marker>
     <inkscape:perspective
        sodipodi:type="inkscape:persp3d"
        inkscape:vp_x="0 : 526.18109 : 1"
@@ -47,8 +73,8 @@
      inkscape:pageopacity="0.0"
      inkscape:pageshadow="2"
      inkscape:zoom="1.4"
-     inkscape:cx="390.80127"
-     inkscape:cy="741.30304"
+     inkscape:cx="367.22984"
+     inkscape:cy="701.30304"
      inkscape:document-units="px"
      inkscape:current-layer="layer1"
      showgrid="false"
@@ -131,11 +157,11 @@
        id="rect3815"
        width="35"
        height="35"
-       x="403.85797"
-       y="299.32153" />
+       x="556.14368"
+       y="179.32153" />
     <g
        id="g4342"
-       transform="translate(383.85797,156.95934)">
+       transform="translate(536.14368,36.95934)">
       <path
          transform="matrix(1.0789474,0,0,1.0779221,-3.7312032,-12.827851)"
          d="m 51.785715,160.04076 c 0,7.59391 -6.076135,13.75 -13.571428,13.75 -7.495293,0 -13.571428,-6.15609 -13.571428,-13.75 0,-7.59392 6.076135,-13.75 13.571428,-13.75 7.495293,0 13.571428,6.15608 13.571428,13.75 z"
@@ -162,12 +188,12 @@
     <text
        xml:space="preserve"
        style="font-size:12px;font-style:normal;font-weight:normal;text-align:center;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Bitstream Vera Sans"
-       x="518.02942"
-       y="319.68921"
+       x="670.31512"
+       y="199.68921"
        id="text3833"><tspan
          sodipodi:role="line"
-         x="518.02942"
-         y="319.68921"
+         x="670.31512"
+         y="199.68921"
          id="tspan3837">Characters</tspan></text>
     <g
        id="g3869"
@@ -1144,15 +1170,15 @@
        x="443.85797"
        y="179.32152" />
     <rect
-       y="299.32153"
-       x="443.85797"
+       y="179.32153"
+       x="596.14368"
        height="35"
        width="35"
        id="rect3984"
        style="fill:#ffffff;fill-opacity:0;stroke:#3f3f3f;stroke-opacity:1" />
     <g
        style="fill:#ffb5dc;fill-opacity:1"
-       transform="translate(423.72715,156.83883)"
+       transform="translate(576.01286,36.83883)"
        id="g3996">
       <path
          sodipodi:type="arc"
@@ -1229,5 +1255,44 @@
        id="path3941"
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+    <path
+       style="fill:none;stroke:#ff0d0d;stroke-width:1.5;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;stroke-miterlimit:4;stroke-dasharray:1.5,1.5;stroke-dashoffset:0"
+       d="m 512.14286,404.50504 -179.28572,0"
+       id="path3040"
+       sodipodi:nodetypes="cc" />
+    <path
+       sodipodi:nodetypes="cccccc"
+       id="path3814"
+       d="m 512.14286,440.50504 -30.82143,0 c -9.87798,0.7619 -2.47024,-32.04762 -10.20536,-32 l -28.20536,0 c -7.75595,-0.7619 -2.65476,32.04762 -10.41071,32 l -99.64286,0"
+       style="fill:none;stroke:#ff0d0d;stroke-width:1.5;stroke-linecap:butt;stroke-linejoin:miter;stroke-miterlimit:4;stroke-opacity:1;stroke-dasharray:1.5,1.5;stroke-dashoffset:0" />
+    <path
+       style="fill:none;stroke:#ff0d0d;stroke-width:1.5;stroke-linecap:butt;stroke-linejoin:miter;stroke-miterlimit:4;stroke-opacity:1;stroke-dasharray:1.5,1.5;stroke-dashoffset:0"
+       d="m 512.14286,476.50504 -179.28572,0"
+       id="path3816"
+       sodipodi:nodetypes="cc" />
+    <path
+       sodipodi:nodetypes="cc"
+       id="path3818"
+       d="m 512.14286,512.50504 -179.28572,0"
+       style="fill:none;stroke:#ff0d0d;stroke-width:1.5;stroke-linecap:butt;stroke-linejoin:miter;stroke-miterlimit:4;stroke-opacity:1;stroke-dasharray:1.5,1.5;stroke-dashoffset:0" />
+    <path
+       style="fill:none;stroke:#000000;stroke-width:1px;stroke-linecap:butt;stroke-linejoin:miter;stroke-opacity:1;marker-end:url(#Arrow1Lend)"
+       d="m 503.09487,367.44494 -41.18975,54.12021"
+       id="path3820"
+       sodipodi:nodetypes="cc" />
+    <text
+       xml:space="preserve"
+       style="font-size:12px;font-style:normal;font-weight:normal;text-align:center;text-anchor:middle;fill:#000000;fill-opacity:1;stroke:none;font-family:Bitstream Vera Sans"
+       x="587.14282"
+       y="346.64789"
+       id="text4458"><tspan
+         sodipodi:role="line"
+         id="tspan4460"
+         x="587.14282"
+         y="346.64789">This tile is tagged &quot;top&quot; and</tspan><tspan
+         sodipodi:role="line"
+         x="587.14282"
+         y="361.64789"
+         id="tspan4462">inherits its Z value from below</tspan></text>
   </g>
 </svg>