How to stitch together multiple files without importing via "image rectification"?

SwoodySwoody Global Mapper UserPosts: 97Trusted User
Hi, is there a way to stitch together referenced nadir jpeg images that are overlapped approx. 60%  without going through the "image rectification" process? If I simply import them, they import as icons at their location, not images. Likely I am not using it right (newbie) but I simply want to load them all, and THEN rectify if needed.  Any suggestions greatly appreciated!  Thanks! Sara

Best Answer

  • MykleMykle Global Mapper User Posts: 410
    Answer ✓
    I would not expect this imagery to match up without additional processing.  The impression that I get from your data is that the top of these images is almost at magnetic south.  (182 degrees "M"agnetic)  So you would need a program that can understand that data and then process the image so the scale and orientation is uniform.   

    It is not easy to suggest which way to go at this point.  The photographic distortion (compared to a map) is minimal when looking vertically with a focal length that is not wide angle.  So you could piece them together, sorta.  It may be easiest to have a base image for reference, like Google Earth, and line up your images one-by-one with a lot of shifting, rotating and scaling. 

    Just be sure to use points on the ground for reference, rather than points on top of a building.  All points above the ground that are not at the nadir will be shifted away from the point on the ground below.  That will be true even for orthorectified images, unless you have good set of LIDAR data and a good 3D ortho program, and probably a lot of work. 

    It sounds like you have a good set of images, with appropriate overlaps.  But they are basically raw data at this point. 

    I'll assume that you don't need to make measurements on these images without further processing.  So you could use something like Photoshop to accumulate the images into one, then place it in Global Mapper and only need one shift/rotate/scale.  I'd find that easier in Photoshop than in GM, so your choices may be different with other image editors.   If your images are large (filesize), Global Mapper is likely to be easier to work with. 

    I'd like to see some suggestions from others about programs useful for orthorectification, whether 2D or 3D.  Some high-end GIS programs are probably able to do these tasks, and the list should include programs with prices more reasonable for individual users.  You are not the first person that is using a UAV for mapping and looking at Global Mapper for help. 

    Cheers,
    Mykle

Answers

  • MykleMykle Global Mapper User Posts: 410Trusted User
    Hello Sara,
    Please remind us what version of GM you are using.

    It sounds like your "referenced" jpegs have one coordinate for the nadir point, and does not include scale or orientation that would be defined by a projection. 

    What you are doing would be appropriate for attaching an image to a point, but that image is not expected to be georeferenced. 

    Perhaps you have a few more details to guide our responses.
    Cheers,
    Mykle
    (SARA: Southern Arizona Rescue Association) hi hi
  • SwoodySwoody Global Mapper User Posts: 97Trusted User
    Hi Mykle,

    Thank you for responding!     I am using a trial of v17.

    I am attempting to take Nadir photos and stitch them together, and have them correctly manifest on the map.

    Does "geo-referenced" always refer to images that have all four corners geo-tagged (?) as opposed to just the center? 

    So, I if I understand correctly, I need to "rectify" each image so in the right position before stitching? Does Global Mapper even do stitching?  I am not clear on this.  Maybe I need to find a software that will stitch them first, then import and rectify?? 

    Thank you in advance for any help, and apologies for newbie questions. Sara
  • MykleMykle Global Mapper User Posts: 410Trusted User
    Hi Sara,

    The simplest reference files use the TFW filename for TIFF files, and use the same format for GIW for GIF, JGW for JPG, and so on.
    Basically, the rectangular (like UTM) coordinates for the upper left corner are specified, and the pixel scale in X and Y directions.  There are additional values that are zero for applications that I have worked with, that can define rotation.
    https://en.wikipedia.org/wiki/World_file

    There is no datum or projection included in these files. 
    A PRJ file can include datum and projection, but it won't (to my knowledge) specify the location of a particular image.  So the info from both files are needed to properly place your image. 

    Some terms are defined here:
    https://en.wikipedia.org/wiki/Image_rectification
    https://en.wikipedia.org/wiki/Aerial_survey

    It sounds like you have aerial photographs that were acquired by a camera flying a defined pattern over an area.  And it sounds like these images were acquired by a commercial survey company.  Further, it sounds like some processing has been done on these images, adjusting them to a vertical view, and adjusting them photogrammetrically so the scale is uniform in all directions, everywhere on the image.  This is not a simple process. 

    There are UAV's that can acquire images, but converting them requires high-priced programs AFAIK.  Global Mapper does not do this (yet).  Basically you need a set of parameters for the camera, and GPS data.  The camera data include focal length, tilt (vertical angle), altitude, and so on.  I'd like to find a simple diagram showing how scale varies across an image where the viewpoint is straight down.  It isn't a simple grid like you would have on a map. 

    And, if your area has a wide range of elevations, another level of adjustments may be required that are even more complex. 

    However, I do not understand why your images include only the coordinates of the nadir point (one point near the center of the image).  We need the scale and orientation of the image as well.  Perhaps that product is more highly priced. 

    If you know or can measure the scale of the images, you could define the coordinates for the top left corner and the pixel scale and manually create JGW files (for JPG files). 

    As a wizard recently said: "It's complicated".
    Mykle



  • SwoodySwoody Global Mapper User Posts: 97Trusted User
     Hi Mykle, 

    Thank for all the detailed info!

    The images were not acquired by a commercial survey company, but by us (we do aerial filming). As a test, I was hoping to use the nadir stills in conjunction with the video. 

    No processing has been done on the files.  I thought I would be able to import and stitch them, then rectify if the imagery is not matching up. 

    I am not sure exactly where the GPS is taking the reading - I assumed it would be the center of the image. The images have quite a bit of metadata (please see attached) in addition to all the embedded camera EXIF data.

    Sara
  • SwoodySwoody Global Mapper User Posts: 97Trusted User
    I am testing via a trial of Pix4D, but that software is ridiculously expensive. So not really a good option.  Thank you, thank you again for taking time to help me with this. I truly appreciate it, Mykle!
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