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Create .prj file for local grid

nrobison
nrobison Global Mapper User
edited November 2014 in Projection Questions
Often in mining and sometimes in civil and surveying projects I'm faced with a local grid system. If I'm lucky "north" is pretty close to "north" in some well-defined projection (e.g. State Plane Grid) in the US. In that case you can survey a single point, enter State Plane scale factor (>1 for higher elevations, similar to "grid to ground"), and then enter "extra false" Easting and Northing.
Save this prj to file and voila, "mine grid" data can be entered, imported at any time, and global information (DigitalGlobe, etc.) can be natively opened within GM. My cup overfloweth.

BUT when the local grid system does NOT line up with any known north, I need help. I found the V13 shift/transform option as a right-click from within Control Center, but what I really want is to enter two or three coordinates (x,y) in both the local grid and true (e.g. NAD83 Lat/Long) geographic coordinates, and let GM create a new projection for me, that can be saved as .prj and used forever to download reference data into the local grid.

Any suggestions?

Comments

  • global_mapper
    global_mapper Administrator
    edited January 2012
    What you can do is create a control point file as you describe of the following format for each point:

    mine_grid_x,mine_grid_y,longitude,latitude

    Then right-click on your layer in the Control Center and specify to shift/transform the coordinates then choose to do it based on a file, then select the file with these control points listed. Then each time you loaded a mine-grid file you would specify it was in NAD83 Geographic (lat/lon), then apply this transformation to actually shift it there. There isn't a way to capture this in a .prj file since the PRJ file format doesn't support such a concept.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • nrobison
    nrobison Global Mapper User
    edited January 2012
    Thank you very much, that will get me close: I can import mine grid data and then export it to a known projection (e.g. UTM). I would eventually like to export known data (like NAIP imagery) as GeoTIFF in mine grid coordinates. Since manually shifting aerial photos is (like almost everything else in CAD) rather clumsy, I think that GM could probably elegantly impliment a "custom .prj" feature. Thank you for your consideration, and as always for the extraordinary software.
  • nrobison
    nrobison Global Mapper User
    edited April 2012
    I found an old thread which included (apparently) formatted text of a custom PRJ file, and that implies that creating one of my own is not impossible. I'll be trying to do that manually (can use rotation deviation from two "bounding" PRJs; one which rotates clockwise of the desired mine projection, and one that rotates ccw) to select a central meridian. Then its just a matter of getting the units to work.

    But in the meantime I wanted to see if GM had already considered a simple "pick three ordered pairs and GM will make a PRJ for you" feature.

    Thanks as always.
  • global_mapper
    global_mapper Administrator
    edited April 2012
    You can't create a projection that way, but you can transform the coordinates of any layer like that. Simply right-click on the layer in the Control Center and select the transform them. You can provide a direct translation with offset, scale, and rotate yourself, or you can also provide a control point file with coordinates in the old and new system and have them transformed that way.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • JustinK
    JustinK Global Mapper User Trusted User
    edited November 2014
    Currently, I use CAD to shift local grid information into proper state plane coordinates and then import into global mapper, which is set up in SPCS or geographic etc. In CAD, I draw the local grid and move it into proper position in state plane. The local grid specifies rotation and 2 points with both local and state plane coordinates. What I want to do is create a prj file based on this information. I realize this created .prj file will only apply to the local area and is not a true “projection” file. However, I would like to be able to send this .prj file to the team working on a job so all information is consistant. I do not want to shift every piece of information from site grid to a built in datum everytime. In other words, I want to add a point to Global Mapper at N,E 100,100 and have global mapper know the associated real world coordinates, and this point 100,100 will show up at the correct spot on the earth.

    How do I solve these parameters so global mapper knows the real world points associated with local grid referenced inputs. I believe it is possible (and only applicable to the specific project site) because we have created a .prj of a local grid before when a surveyor provided all transformation information in a report, but I do not have this information on the current project.
  • DanHG
    DanHG Global Mapper User
    I figured a work-around for this, at least for the data from our mine grid. What you'll need to know is the origin in lat/long and the rotation of your grid from true north. Then use Transverse Mercator projection, enter the values as needed to make it work.

    You'll have to do a bit of math to figure out the rotation. In my case I was given the rotation from UTM to mine grid, but to use TM you have to also factor in the rotation between UTM and true north at your origin. This is easily done by setting GM to UTM and drawing a line with ortho snap on. Then just check it's bearing.

    I used a scale factor of 1. Our grid is simple, true distance, local plane coordinates over a distance of about 7 km maximum (so the error is small). If your origin is far from the area of interest I'd suggest getting the UTM and/or Lat/Long of something central to your site, then using the false northing/easting to set the coordinates. Rotation might be trickier in that case, I'm not sure.

    Bit of trial and error and you should be able to make it work. Remember to save your projection to a file before you change it back to something else lest you have to enter all the values again.

    GM is a fantastic tool for managing multiple data sources as is, but I have to agree with the OP, it would be handy if there were a simpler way to define local coordinates. I started with this post, some frustrated reading on Wikipedia, and about 2 hours of futzing, then about another hour of checking different data sources to see if they lined up. It would certainly be handy if there were a best fitting tool using pairs of local grid coordinates and coordinates in a different datum such as any of those in GM.

    Also, it would be good if one could change the elevation in a custom local projection. We have a local false datum of -3048 m to keep our whole mine in positive elevation coordinates. It's a pain to have to offset incoming data sources that are all in geodetic coordinates.

    Thanks!

    PS I'm using GM v15.2, my company is in the process of upgrading to 16, should be rolled out any day.
  • Hello All,

    Blue Marble's Geographic Calculator application has a 'Best Fit' tool, which will relate a local coordinate system to a known projected or geodetic system using control pairs. You will not be able to create a PRJ for the local grid system, but once the Fitted system is established, you can transform local grid data to other projected and geodetic systems that are supported by GIS and mapping applications.

    The Best Fit tool is a very specific/unique tool and is almost exclusively used by those working with local mining grids. If you would like to evaluate the Best Fit tool in the Geographic Calculator, please contact Blue Marble's Sales (sales@bluemarblegeo.com) or Support (geohelp@bluemarblegeo.com) team(s).

    You will need to have at least 3 control pairs to create a Fitted system, there are a number of Math transforms available - which you decide to use depends on the number of control pairs, and the extent of the grid you are creating the fitted system for.

    Please feel free to contact Blue Marble Application Support directly if you have any questions.

    Regards,

    Jasmine B.
    Blue Marble Application Support
    http://www.bluemarblegeo.com/support/support.php



  • nrobison
    nrobison Global Mapper User
    Nice to see this discussed again - we developed an iterative spreadsheet process for the transformation, starting with a the base projection (Mercator, etc) that is used for the local state plane or UTM, whatever is closest to "north" because distortions increase with distance to central meridian. After setting central meridian, it is mostly a matter of messing with the scale factors and false northing & easting - we have it down to within the accuracy of the base data: 0.1 foot for a well-controlled mine. I do want to point out to Blue Marble that the custom .prj is almost obligatory, as discussed above (sharing with clients, etc). I'll evaluate the Best Fit tool gratefully, but can't wait for the extension that creates a PRJ within GM!
  • This is exactly what I am looking for as well.  This is a pretty common thing and makes sense that a program like Global Mapper would be able to do.
  • JoeK
    JoeK Global Mapper User

    Helpful - But I have a slightly different problem.  I'm trying to convert several ECWs in UTM to Local Coordinates.  Unfortunately, the Image Rectifier dialog is forcing Longitude and Latitude onto the local coordinates, even though I specified local coordinates into the 'Projected X' and 'Projected Y'. 

    Any ideas on how to force Global Mapper to accept local coordinates and disable the Longitude & Latitude?

  • nrobison
    nrobison Global Mapper User
    Joe, I don't think you are going to get there, because lat/long (that is, a real place on the globe) is sort of a pre-requisite to having a GIS at all. You can't have an entity NOT be on the sphere. For that, one must use CAD or the "Fake coordinates to allow viewing" kludges available in GM. 
    Once you have the UTM or local coordinate entities imposed on the sphere, you can freely convert them from one to the other. There is a batch convert/reproject tool which is great if you have known projections to work to or from. 
    But Lat/Long is obligatory, even if it is wrong (and its best to work with it right)
  • saeed.akhtar@hotmail.com
    saeed.akhtar@hotmail.com Global Mapper User

    I am looking for to convert the point cloud data in (lat/long/elev) to local grid system. Is there a way to do this conversion in GM? I have couple of control points and how many control points should I use?

    Please advise.

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