GIS users analyze relationships among real-world objects. Features must be georeferenced so data can be correctly related to locations on the earth's surface. Georeferencing data is accomplished by assigning a coordinate system - a reference system used to locate geographic features on a two- or three-dimensional surface. As ArcIMS services become more accessible, and many users want to combine data from different services, identifying a projection for your service becomes critical. If a service has an undefined projection, many applications may not be able to integrate the layers correctly. You also need to define the projection if you plan to create WMS or WFS Services.

Since Author does not provide a way for you to add elements to support georeferencing, you must add them manually. The projection elements are:

• FEATURCOORDSYS: coordinate system to which layers in an ArcIMS service are projected.
• FILTERCOORDSYS: the current coordinate system of filters in the requesting client.
• COORDSYS: defines the coordinate system of a layer.

All of your ArcIMS services should include FEATURECOORDSYS and FILTERCOORDSYS. In a map configuration file, these two elements should have the same value. If you are unfamiliar with the projection elements in ArcIMS, it is highly recommended that you read Using Projection Elements to better understand the relationship of these elements.

Adding projection elements

Layers are missing from a map after starting a service

• Check if the missing layer has a *.prj or spatial references table. If not, try adding COORDSYS to the layer.
• If the layer is a raster layer, check if is in the same coordinate system as the service. If not, add COORDSYS to the layer.

Some features are missing on one or more layers. The missing features are generally around the periphery of the layer.

Try using DENSIFY.

<LAYER type="featureclass" name="Countries" visible="true" id="1">   <DATASET name="countries" type="polygon" workspace="shp_ws-0" />   <COORDSYS id="4326" />   <DENSIFY tolerance="100" />   <SIMPLERENDERER>     <SIMPLEPOLYGONSYMBOL fillcolor="255,255,153"/>   </SIMPLERENDERER> </LAYER>

The process of data densification adds virtual points to a layer before the layer is projected. In some cases, the number of points defining the geometry in the original layer is not enough to describe the geometry correctly in the projected layer. DENSIFY can be used on a layer to define how much data should be densified before the layer is projected.

A word of caution about DENSIFY: if the tolerance value is too small, the Spatial Server can slow down considerably. It is better to start with a large number for the tolerance. A guideline is to start with a value about 20 to 30 percent of the distance between the minimum and maximum x-coordinates of the current map extent.

A request to the service works the first time but does not work correctly from the second request onward.

Check if FILTERCOORDSYS and FEATURECOORDSYS are the same value in the map configuration file. These values must be the same. Only in a request can they be different.