6

This is not precisely GIS problem but still spatial.

I would like to measure certain features of a thin wood slice. I have a scan of that slice.

What coordinate system should be used to obtain a view "just as it is" and to be able to measure features (by ruler tool or by calculating geometry in ArcMap) of a object from scan?

Is that even possible in ArcMap?

Example of a scan:

enter image description here

  • 4
    a Cartesian coordinate system should suit your needs – risail Dec 7 '16 at 19:19
2

Knowing the resolution helps. If you have access to Python and GDAL, you can do something like (not run):

from osgeo import gdal

# Read the original file
fn = "c:/fullpath/myfile.tif"
ds = gdal.Open(fn)

# Write out a copy, changing the GeoTransform
driver = gdal.GetDriverByName("GTIFF")
ds_out = driver.CreateCopy("c:/fullpath/out.tif", ds, 0)
ds_out.SetGeoTransform([0, ds.RasterXSize / 2400, 0, 0, ds.RasterYSize / 2400])
ds_out = None
ds = None

The key part is the GeoTransform, it is [xorigin, xpixelsize, 0, yorigin, 0, ypixelsize].

Having the proper origin (0,0) in the upper-left, and a physical pixel size (in inches), you will be able to make relatively accurate measurements. You don't need to set a projection when opening it ArcGIS. It will show as unknown units, but you know that they are in inches. Having a ruler or other known measure in the picture would help confirm that everything is as it should be.

The simpler alternative is to do any tracing / analysis in 1x1 unit pixels, as it is in the raw image, then convert distances and areas appropriately using the known resolution (1 pixel = 1/2400 inches, for distances, in your case).

If you need to assign a projection, something like this "+proj=eqc +ellps=sphere +units=int-in" might work:

PROJ.4 : '+proj=eqc +lat_ts=0 +lat_0=0 +lon_0=0 +x_0=0 +y_0=0 +a=6370997 +b=6370997 +units=m +no_defs '

OGC WKT :
PROJCS["unnamed",
    GEOGCS["Normal Sphere (r=6370997)",
        DATUM["unknown",
            SPHEROID["sphere",6370997,0]],
        PRIMEM["Greenwich",0],
        UNIT["degree",0.0174532925199433]],
    PROJECTION["Equirectangular"],
    PARAMETER["latitude_of_origin",0],
    PARAMETER["central_meridian",0],
    PARAMETER["false_easting",0],
    PARAMETER["false_northing",0],
    UNIT["int-in",1]]

I don't think it really matters what you pick as long as the units and pixel size are correct, because you are dealing with such a small area, geographically speaking.

  • I scanned a page of squared paper and it seems that my "unknown units" are already inches and I can make accurate measurements with ruler. Also, when I create new polygons they have measured area, so my problem is partially solved. However, with unknown coordinate system, there are some operations that I can`t perform (ex. Calculate Geometry). Any ideas how to obtain "full functionality"? – Kubson Dec 8 '16 at 13:08
  • See my edits. I would think that as long as the units are set, something like the Equidistant Cylindrical (Plate Carée) projection should be fine. It will position the image at the equator and prime meridian, and it is so small that there should be no effects from the datum/ellipsoid, regardless of the selection. – Benjamin Dec 8 '16 at 14:46
9

Use a Cartesian coordinate system you can set 0,0 arbitrarily (ie: Center Of Gravity or a corner. The units can be anything and any math you want to apply will require elementry level geometry skills.

You will want to georectify the *.tiff here is an example of how to do it there are many other tutorials if you google georectify tiff this is an esri article Fundamentals of georeferencing a raster dataset .

If you do not have real world coordintates you will need to start from 0,0.

  • I probably should have put that in a main question, but how can I apply that in ArcMap? Should I create a new coordinate system? – Kubson Dec 7 '16 at 20:02
  • Can you edit your question to describe what you have and where you want to go. Do you have the shape yet? What are you trying to accomplish? Whats driving the decision to use arcmap? Can you use something else? arcmap is not the right tool for drafting if thats all you need to accomplish. – risail Dec 7 '16 at 20:08
  • I edited the question. Later I would like to vectorize those hole-structures (vessels) and obtain their area and do some spatial analysis. That is why I would like to use arcmap. – Kubson Dec 7 '16 at 20:21
  • whats the data format of the "scan" – risail Dec 7 '16 at 20:22
  • 1
    It is in TIFF. The resolution of a scan is 2400dpi. – Kubson Dec 7 '16 at 20:25

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