# TB two points calibration question

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Hi all,I

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Hello Francis,Would you please indicate the exact values that you are using? This would make it easier to help you.Best regards.Luis

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Hello Luis,There you hve:First point 28,44189 -13.864120Second one 28.46354 -13.863405I get a negative (minus) scale value.I hope these where the values you asked for.Thanks,Francis

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Latitude and Longitude nomenclature??Also, how big is your image? And can you sketch out approximately how you are entering these, i.e.:------------------------------....A.....................B-------------------------------Where A and B are your coordinates above, Lat and Long.When dealing with -ve coordinates, one of the most common errors is that you may be specifying a smaller number as a larger number, and the calculations thus get reversed and you get -ve scale values. Check and make sure you're not doing this.

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Hi,I got the coordinates in DMS and converted to decimal, so I enter the decimal value in the two points. Just confirm I could enter the coordinates as DMS in the two points calibration dialogue?I resampled the image to get 4.8m/pix so the dimensions are 9729x8962Yes I checked the lat and -long values several times.Could it be the problem that the two points are almost in the same longitude?Thanks for your helpFrancis

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>I got the coordinates in DMS and converted to decimal, so I >enter the decimal value in the two points. Just confirm I >could enter the coordinates as DMS in the two points >calibration dialogue? As far as I remember, yes. I'll double check and let you know if NOT.>>I resampled the image to get 4.8m/pix so the dimensions are >9729x8962 Should be OK. Any problems displaying?>>Yes I checked the lat and -long values several times. >>Could it be the problem that the two points are almost in >the same longitude? Definitely. That's why I asked you for the "diagram". I think this could be the problem. Enter somethig "away" from each other (as in my diagram) and see what you get.

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I love the 2 point method, and I agree the runways make the best reference, BUT...when the angle of the rwy is too much n/s or e/w it won't work. Here's my workaround, hopefully it will help you. My premise is that the cellxdim and cellydim values are different for a given photo resolution based ONLY on latitude. Stated differently, for every ring of latitude the distance in units of deg/pixel should be consistant if the resolution of two photos is the same.So...As I work with applying usgs photos to the ms world, it doesn't really matter to me if I derive my cellxdim from one photo and cellydim from another, as long as the resolution of the photos is the same, and the latitude is the same.So, I look for a runway at the same latitude as mine, but with a direction of E/W for instance, and work with a photo of that runway to define cellxdim, and use my real photo of my runway (running n/s) to define cellydim.Hope this helps.Bob B

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Hello all,Thank you Bob, that is, TB doesn

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It's not that TB "doesn't like" the points aligned N-S or E-W.The reason for this is that, for precise calculation, you need a separation of points in both directions. The smaller the separation is, the less precise the calculations will turn out to be (you may even hit singularity "divide by zero" if the points are the same in one direction)

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yes, as Misho posted, the problem is one of math, the two point method subtracts the distance in degrees of the two points (both latitude and longitude), and divides that distance by the number of pixels (horizontal and vertical) between the same two points.When the runway is NS (or EW...same problem for cellydim), then the equation for cellxdim is divided by 0, as there is 0 difference in pixel count in the x direction. And as the runway begins to have some angle to it you do begin to get some denominator, but tiny numerators divided by tiny denominators will only result in a valid ratio if there is no error...not normally the case in fs! So you want a pretty good angle to get a good value.Bob B

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