WMS and Other Images Coming in Grid Units? Setting/Correction?

Question

Greetings- 
 My office commonly has the opportunity to work with our civil designs overlaid on aerial images. This has brought three questions to light... 
 What we often find is that images come in close to the design but not right on; this occurs with pretty much all images on WMS servers and also on many images we obtain via download from local agencies (city or county). We have determined that the error is due to grid vs. ground. 
 The images from more local sources typically have world files that control the layout of the image. The images are typically UTM, so we tell MicroStation to use the sister file coordinates and to assume the sister file is in meters. We also select the coordinate system of the DGN file (usually a state plane or local system in iFeet). MicroStation can then work out the difference and position the image correctly. Except... it's in grid units and not ground units. So the image is almost correct. 
 In SS2, there doesn'tseem to be much that could be done to fix this. In SS3, a new Coordinate System Modifier appeared - we are able to select the Helmert transform, apply our ground area factor (GAF), and make the image come in exact. First question: is this the best way to accomplish this correction, or is there something else we might use? Second question: is there any way to accomplish this correction (at all) in SS2? 
 For WMS images, there seems to be no way we can apply this correction. Third question: is this the case? if so, what are our options? 
 Thanks!

Alain Robert · Accepted Answer

Hi Jcallicott, 
 You are right to assume that the Helmert post-transformation was introduced, in part, to enable application of a ground to grid scale factor and it is the proper way to do it. The only alternative possible that may be used with SS2 is applicable only when the Coordinate System definition is based upon Transverse Mercator or Lambert Conformal Conic projection methods. Bentley offers variations of these two methods where post-Affine transformation can be specified; this implies you introduce new coorddinate system definitions using Bentley Map/Power Map into a user-defined coordinate system dictionary. Other projection methods can sometimes be modified by changing the false Easting, False Northing and scale reduction factor to apply the proper correction, but I would not recommend this alternative. 
 If the use of Transverse Mercator with Post-Affine or Lambert Conformal Conic with post-affine is selected, keep in mind that such projection methods are not standard and cannot be exported to most standard file format such as ESRI Shape Files with PRJ/WKT sister file or GeoTIFF Raster file. WMS servers would not understand and even less publish data using such Coordinate System definitions. 
 I would recommend that all data be stored using GRID coordinates in all cases (including WMS published data) and that when Ground coordinate are needed then the Active Model of the DGN file be set using the appropriate Coordinate System setting (with grid to ground correction). This should allow input and output using ground values. 
 If your data is stored as ground coordinates (such as WMS or some georeferenced raster file) then the Helmert transformation cannot be used as such post-transformation cannot be stored into raster files. I did notice that the coordinate system of the raster attachement itself cannot be decorated with a Coordinate System modifier such as the Helmert Local Transform. I will verify such restriction is by design and it is likely so, again because standard file format do not support such modifiers. 
 Regards, 
 Alain Robert