Georeferencing – Should I Update or Rectify?

Once you have finished choosing and evaluating your links, you are ready to persist your results. There are 2 options to choose from: Update Georeferencing and Rectify.

Update Georeferencing

  • Only updates your files- no redundancy created
  • Update are almost immediate

Rectify

  • A new raster dataset is created and the original file is left unaltered
  • Choose many options: such as file format, cell size, resampling technique, and NoData value.
  • This is the only option for some files: such as GRID and other files that cannot be updated (e.g. read-only files)

Depending on your workflow, both Update Georeferencing and Rectify have their advantages.

This entry was posted in Imagery and tagged , , , , , , , . Bookmark the permalink.

Leave a Reply

4 Comments

  1. gionette says:

    We’ve found that if you are making an image catalogue with the georeferenced images, you need to rectify or else if doesn’t work properly. Also I find that rectifying is necessary if you might need to use that image in another map document, because I’ve found that sometimes when I’ve updated the georeferencing, I’ve had visibility issues when I’ve tried to bring it into another arcmap document. On the other hand, after Version 9.0, we’ve found that a big annoying , pain in the behind, black box gets added to rectified rasters. Now if you can get rid of that you did in earlier ArcMap versions a lot of users would be thrilled!!!

  2. Simon Woo says:

    If you meant raster catalogs (within a geodatabase), did you create a managed, or unmanged raster catalog? Referenced (Unmanged) raster catalogs should be able to pick up the new georeferencing image georeferencing. A Managed raster catalog on the other hand stores the rasters in the geodatabase, so you would actually have to modify those rasters, not the original.

    If you meant DBF style image catalog, then those are always referenced (unmanaged). However those image catalogs required you to specify the minimum bounding rectangle for the rasters in advance, so any update to the source georeferencing will not matter. You should update the georeferencing first, and then create the image catalog (with the new proper extents).

    In regards to the black border, if you specify a NoData Value while you Rectify (i.e. 0), or if you specify Grid as your output format, your output should not have a black background. I believe previously we automatically output your data to a Grid format. ArcGIS 9.x uses IMG and TIFF as the default output formats.

  3. gionette says:

    the only problem with the grid is that it’s pretty much useless anywhere else BUT I do a lot georeferecing of black and white documents and If I set no data to 0 it makes it all black but If i set no data to 1, OMG it makes it wonderfully perfect!!! Thanks so much for the tip. I’ve been looking and have asked a few people *cough* tech support *cough* and never got this answer! You have saved us a lot of arcscan work!!! Thanks Simon!!

  4. griner says:

    I see no point in rectifying if you’re using the default affine (1st order transformation) georeferencing transformation. This is a transformation that is allows the image to be translated/scaled/skewed/rotated quickly by software and doesn’t require the raster to be re-sampled. It does require that once the raster file has its transformation parameters written (e.g. a .tfwx file for tiff rasters) that the raster file must not be subsequently edited in a way that changes such properties as raster-rows, raster-columns, or rotate/flips. But the same is true for rectified/resampled images – the image raster size/orientation must not be change outside of ArcGIS. Rectifying images as a result of georeferencing entails all the issues of image resampling: cell size, bit depth, pixel resolution, raster format (tif, bmp, grid, png, etc.).

    Creating a library/directory of georeferenced images that are simply ‘updated’ (by writing out the transform parameters file) and then marking the images files as read-only should suffice for having a working library of georeferenced images without having to make rectified/resampled copies of all the images in the library. This will not require users to add the image to their map and then have to load the correct ‘links’ file to get it to georeference correctly.

    However, for non-affine projections, the GIS is not able to record a file of transformation parameters. Thus, the user would be required to not only load the correct links file with the georeferencing toolbar, but to also select the correct type of transformation those links work for. In this case, rectifying/resampling the image may make sense to avoid mistaken mismatched links/transformations.

    Depends on your file management process and shared images practices.