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iShare returns results based upon a location - whether that be a click on a map or a given address in My House. If you imagine that a point click equates to a small bounding box on the earth's surface then tolerance is the measure by which we can expand that bounding box.
iShare Maps / GIS Pixel Tolerance
Info Click results are returned based upon the following: when a user clicks on a map they are actually selecting an individual pixel. iShare takes that pixel and with a Scale a bounding box is calculated. Any geometries found to overlap with this bounding box are returned as results. Unfortunately for point layers we might not click on the exact pixel that covers the point's geometry. For example a layer may be rendered with a 20px symbol image for each point object. The symbol is anchored to the point's geometry by the image's mid-point by default. This means that if a user doesn't click on the exact centre of the symbol they will not hit the actual geometry of the object. In order to correct this a map pixel tolerance is set which widens the bounding box to cover the physical area that the symbol represents.With no tolerance a user may click on a point layer and may not hit dead centre on a point's symbol so no results are returned. This is because the symbol is rendering a greater area than what the geometry covers. A tolerance will widen the search area to include the associated point's geometry. Likewise if we take an residential property that is situated next to a neighbouring ward we want to use no tolerance as widening the bounding box will return incorrect results.
iShare Maps / GIS Pixel Tolerance
Check that the iShare Settings for MapPixelTolerance is reasonable. What is reasonable is based upon the size of all point symbology used by the .map file. Open Studio and select Settings, on the General Settings tab scroll down until you see the Web entries. Here you will see the MapPixelTolerance setting. This is the number of pixels the bounding box is calculated to cover at any one scale.
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The In general the MapPixelTolerance should never be set to less than 10 pixels and probably no more than 20. Lower than 10 and Point symbols may not return results. Higher than 20 and the bounding box used becomes too large and more results are returned than expected. The As said, the value is dependent upon the size of the rendered symbols used and the anchor position for the each point layer. The majority of points will use a mid-point anchor with a symbol image normally no bigger than 20 pixels. In that case '10' would be an appropriate value to use. Unfortunately there is a danger when clicking close to Polygon boundaries more results than desired may be returned. This is something that we are looking to resolve in future versions. |
My House Precision
For Point layers When using a point layer in My House the Find Nearest setting is always used and . This defines a Distance, specified in metres, which equates to the desired tolerance. For Polygon layers you need to use the Show My setting and specify TOLERANCEUNITS as is used instead. Make sure that TOLERANCEUNITS equals METERS in the LAYER definition in the .MAP file. The tolerance ( distance ) is hard coded at 0.01. So the The result is that a one centimetre bounding box is used around for the address locationgiven location. Make sure that TOLERANCEUNITS METERS is defined for each polygon layer in My House as the default units is PIXELS which will can produce incorrect results.
My Nearest Precision
These should always use the Find Nearest setting for both Point and Polygon layers where the Distance, specified in metres, equates to the tolerance.