Latest Contributions by MelitaKennedy

Hi kris, Check out Geoscience Australia. They have some free data downloads and information on the coordinate systems in use. The current geographic coordinate system is GDA 1994, with earlier ones, AGD 1966 and AGD 1984. There are UTM-like zones, "Map Grid of Australia" aka MGA, based on GDA 1994.  There are a few national projected coordinate systems too, plus some state-based ones. Check state GIS sites for data too. Melita ... View more

Supposedly, I am a coordinate systems expert, and it does look like there's a missing transformation. Both posted shapefiles are still using British National Grid (based upon OSGB 1936). If I add "Imagery" basemap which is using WGS 1984 Web Mercator, then add the shapefiles, I get the warning message about different geographic coordinate systems. I then set a transformation from OSGB 1936 to WGS 1984 using: OSGB_1936_To_WGS_1984_Petroleum I don't know if this transformation is available in the version you're using. When I okay that dialog and compare the Shannabank Hill, Fort poly/point against the imagery, I can see that they're overlaying within a few meters. If I use OSGB_1936_To_WGS_1984_1 (a not-as-accurate transformation), the offset is more, closer to 7-8 meters as opposed to 4m. When you converted the shapefiles to WGS84, did you use the Project Tool or the Define Projection tool or the data's property page in ArcCatalog? The latter two update the coordinate system information only, they don't change the coordinate values to the 'new' coordinate system. Melita ... View more

Hmm, there's no datum transformation, and both use close central meridian values (13 30 versus 15), so I wouldn't expect that to be a problem. Because they're rasters, you might want to try setting a registration point in the Project Raster tool. Because you're trying to match a target raster, I would use its lower left corner as the registration point. The raster that you're projecting does not need to actually include that point. Melita ... View more

Hi Jonas, Is the 5m offset in the XY values or in the Z values? The project raster tool will not affect any vertical values at this time. If it's 5m in XY, what are the cell sizes of the two rasters? Melita ... View more

Hi Sam, Now I'm wondering if there's something odd in the Mercator coordinate system definition. Offshore south Wales, if the data's using Esri's "WGS_1984_World_Mercator" should have easting values around -500k to -600k and northing values between 6.6 and 6.8 million. Is that what you're seeing? If you add the raster to ArcMap, but set the data frame's display units (data frame property page, general tab) to decimal degrees, do the values make sense? If I convert to BNG using the 7par transformation, the easting values are around 150k to 260k and northings around 175k to 270k. The output values you're reporting do not look like BNG, which means something odd in the source coordinate system. Melita ... View more

Hi Serge, You may be able to save as dbf using Access rather than Excel. Melita ... View more

Hi Sam, How far off is the 'best' transformation compared to your test point? All these transformations are built for different purposes and have different accuracies. There's another possibility (not supported in ArcGIS yet, so it's a little more work). Ordnance Survey has released an NTv2 file for the UK that's built from their OSTN02 transformation data and method. You can get it here: http://www.ordnancesurvey.co.uk/oswebsite/gps/osnetfreeservices/furtherinfo/ostn02_ntv2.html After downloading the files, extract them. You'll need to locate the pedata folder in your ArcGIS installation. In that folder, you'll see an ntv2 folder. You can either put the .gsb file there or create a uk folder and use that. Now use the Create Custom Geographic Transformation tool in ArcToolbox to define a transformation for it. For the input GCS, use OSGB 1936. For the target, use WGS 1984.  Method will be NTv2. For the parameter value, if it's just in the ntv2 folder, use the file's name without the extension. If it's in the uk folder, use uk/filename. Melita ... View more

Hi Liz, That's interesting. Unless the original Albers definition is corrupt, it should convert fine. If you look at the raster's xy coordinate system in ArcCatalog, what parameters and linear units does it have? The converted polygon data should use the same coordinate system as the original raster. If the Albers definition is using meters, while the State Plane is in feet, that will definitely offset the values beyond the fact that it's also using Lambert conformal conic. Melita ... View more

Hi Susana, If you don't see WGS_1984_To_Ocotepeque_1935_MB, then use the RN one. It's 7 parameters and should be more accurate than the 3 parameter version. Melita ... View more

Does the raster have a defined coordinate system? If it doesn't, you should assign it the coordinate system that it's georeferenced to, not what you want it to be using the Define Projection Tool or the raster's property page in ArcCatalog. The polygon data should then inherit that information. Melita ... View more

Hi Sam, Oh yes, that's a generic Mercator definition (centered at 0,0) that we added as a test case. If I add WGS84 data to ArcMap and set the data frame's coordinate system to British National Grid, I get this list of transformations with the areas of use and accuracies added: OSGB_1936_To_WGS_1984_1  "UK - Great Britain; Isle of Man" 21.0 OSGB_1936_To_WGS_1984_2  "UK - England" 10.0 OSGB_1936_To_WGS_1984_3  "UK - England; Wales; Isle of Man" 21.0 OSGB_1936_To_WGS_1984_4  "UK - Scotland" 18.0 OSGB_1936_To_WGS_1984_5  "UK - Wales" 35.0 OSGB_1936_To_WGS_1984_NGA_7PAR  "UK - Great Britain; Isle of Man" 21.0 OSGB_1936_To_WGS_1984_Petroleum  "UK - Great Britain; Isle of Man" 2.0 When checking for possible transformations, only the geographic coordinate systems matter. Melita ... View more

Is the raster in WGS_1984_Web_Mercator or WGS_1984_Web_Mercator_Auxiliary_Sphere? That will make a difference. The first one uses a custom geographic coordinate system (modified WGS 1984, "GCS_WGS_1984_Major_Auxiliary_Sphere) and there's no direct transformation between it and OSGB 1936. You have to use two transformations, which is...difficult...in ArcMap, but can be done by converting the data in ArcToolbox using the Project or Project Raster tools. The 2nd one uses standard WGS 1984 and there should be available several transformations. Melita ... View more

A few things to check: What are the units of the CAD data? Sometimes the units are scaled to use centimeters or some fraction of an foot (or inch). If that doesn't match what the coordinate system is using, the data won't be in the correct location. Also ask the engineer if the CAD data is using grid or ground coordinates. To successfully use one of the predefined coordinate systems in ArcGIS, it must be using grid coordinates. Ground coordinates are scaled. It's usually possible to define a custom system to take that into account. Is the CAD data really using the coordinate system that you think it is? Many surveying projects use a 'local' coordinate system with an origin in the project area, some angle to north, etc. It can be more difficult to figure out a custom coordinate system in this case. Sometimes you have to use the georeferencing toolbar to get the CAD data to align with reference data (at which point the CAD data is using the same coordinate system as the reference data. Melita ... View more

Also, Esri does not support the TOWGS84 keyword, so you cannot rely on it to include a datum transformation. Melita ... View more