KML でそれを定義する方法は、hotspot タグを使用することです
<hotSpot x="0.5" y="0.5" xunits="fraction" yunits="fraction">
Specifies the position within the Icon that is "anchored" to the <Point> specified in the Placemark. The x and y values can be specified in three different ways: as pixels ("pixels"), as fractions of the icon ("fraction"), or as inset pixels ("insetPixels"), which is an offset in pixels from the upper right corner of the icon. The x and y positions can be specified in different ways—for example, x can be in pixels and y can be a fraction. The origin of the coordinate system is in the lower left corner of the icon.
x - Either the number of pixels, a fractional component of the icon, or a pixel inset indicating the x component of a point on the icon.
y - Either the number of pixels, a fractional component of the icon, or a pixel inset indicating the y component of a point on the icon.
xunits - Units in which the x value is specified. A value of fraction indicates the x value is a fraction of the icon. A value of pixels indicates the x value in pixels. A value of insetPixels indicates the indent from the right edge of the icon.
yunits - Units in which the y value is specified. A value of fraction indicates the y value is a fraction of the icon. A value of pixels indicates the y value in pixels. A value of insetPixels indicates the indent from the top edge of the icon.