# Georeferencing/Mapping Latitude/Longitude coordinates to a scanned map-image (I DON'T know Lat/Long of Point 0,0)

Hello all you GIS experts!

This question has been asked serveral times: How can I convert Lat/Long data to X/Y or Left/Top values?

All answers (all but one) started with "first you need to find out the Lat/Long values corresponding to your 0,0 point of your image". That's my problem: I have the Lat/Long values of three points spread over the map. I DO NOT have the Lat/Long of the top/left or top/right or buttom/left or buttom/right pixel.

Then I found 2 postings of Mr. imarshad, outlining how to calculate the Lat/Long of Pixel 0,0 from 3 different points with known Lat/Long:

http://www.experts-exchange.com/Databases/GIS_GPS/Q_20955769.html (in short)
http://www.experts-exchange.com/Databases/GIS_GPS/Q_21213084.html (very detailed)

=========================================================
> Here is a brief extract of the first question (the short one):

To georeference Images(in BMP , TIFF and JPG) you need to know at least three points......

First step is to note the X,Y of the three points for which you have Lat/Long information. This can easily be done by using
any Image Package(Photoshop, Paint etc.).
Now take the point with the Least Longitude and name it as point 1 having (x1,y1) and (long1,lat1) .
Take the point having the Max. Longitude and name it point 2 (x2,y2) and (long2,lat2)
and the point having Min. Latitude as point3 (x3,y3) and (long3,lat3)
similarly Max. Latitude as (x4,y4) and (long4,Lat4)

Now your delx ( deg./Pixel in X direction) will be
delX= (long2-long1)/(x2-x1)
similarly
delY= (lat4-lat3)/(y3-y4)

Now the only thing left is to calculate the Lat/Long of any reference point. So we take the Top Left point as the Refernce point........
It will be calculated using

lat0= Lat4+y4 * delY
long0= Long1 -x1 * delX
=========================================================

I did the calcs 3 times now. The values I get, which are supposed to represent the Lat/Long of the 0,0 point on my map-image (lat0 and long0), look weired - or rather completely wrong.

I have a city map of a town in Germany. It is based on a Map & Guide map, so it should be pretty accurate. Its dimensions are 2544 x 4176 pixel.

Here is the geo-/pixel-info of the 3 points that I have:

P1:
x1=478
y1=329
Lat1=50.983069° (50°58'59.050'' N)
Long1=11.321775° (11°19'18.390'' E)

P2:
x2=247
y2=1522
Lat2=50.971017° (50°58'15.666'' N)
Long2=11.318177° (11°19'5.436'' E)

P3:
x3=1813
y3=1208
Lat3=50.973977° (50°58'26.316'' N)
Long3=11.342885° (11°20'34.386'' E)

This was my calc:

delX = (long2 - long1) / (x2 - x1)
delY = (lat4 - lat3) / (y3 - y4)
delX = (11.342885 - 11.318177) / (1813 - 247) = 0.024708 / 1566 = 1.577778 ( <- WRONG?)
delY = (50.983069 - 50.971017) / (1522 - 329) = 0.012052 / 1193 = 1.010226  ( <- WRONG?)

I assume my delta X/Y (delX / delY) is already incorrect.

lat0 = lat4 + y4 * delY
long0 = long1 - x1 * delX
lat0 = 50.983069 - (329 * 1.010226) = -281.381285 ( <- WRONG!)
long0 = 11.318177 - (247 * 1.577778) = -378,392989 ( <- WRONG!)

Someone please help! (imarshad, u there?) :/

Thank you all!

- Jessica
Asked:
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Commented:
Assuming an affine transform from x,y to lat,long

Lat = a*x + b*y + c
Long = d*x + e*y + f

where
a = ((y1-y2)*(Lat2-Lat3)-(y2-y3)*(Lat1-Lat2))/((x2-x3)*(y1-y2)-(x1-x2)*(y2-y3))
b = ((x1-x2)*(Lat2-Lat3)-(x2-x3)*(Lat1-Lat2))/((y2-y3)*(x1-x2)-(y1-y2)*(x2-x3))
c = Lat1 - (a*x1 + b*y1)

d = ((y1-y2)*(Long2-Long3)-(y2-y3)*(Long1-Long2))/((x2-x3)*(y1-y2)-(x1-x2)*(y2-y3))
e = ((x1-x2)*(Long2-Long3)-(x2-x3)*(Long1-Long2))/((y2-y3)*(x1-x2)-(y1-y2)*(x2-x3))
f = Long1 - (d*x1 + e*y1)

If
x1=478
y1=329
Lat1=50.983069
Long1=11.321775

x2=247
y2=1522
Lat2=50.971017
Long2=11.318177

x3=1813
y3=1208
Lat3=50.973977
Long3=11.342885

then
a = -1.4121e-07
b = -1.01317e-05
c = 50.9865

d = 1.57858e-05
e = 4.04847e-08
f = 11.3142

and
x=0,y=0 would correspond to
Lat = -1.4121e-07*0 + -1.01317e-05*0 + 50.9865
Long = 1.57858e-05*0 + 4.04847e-08*0 + 11.3142
Author Commented:
Hi OZO!

Thanks for your quick response!

So now I have Lat/Long corresponing to Point 0,0 on my map. Getting closer .. :D

After all I will get Lat/Long coordinates from a GPS receiver and have to display the location on a map. So I need to know how to convert from Lat/Long to X/Y rather than the other way around ...

Quote from imarshad's solution:
"Now (Lat/Long) of any point on the image can be mapped on the image using
X= (lat0-lat)/delX
Y= (long-long0)/delY"

Our lat0 is 50.986469 and long0 is 11.314216. (I used your equations to redo the lat0/long0 clac with more digits kept)

The deltaX and deltaY I calculated with imarshad's equations seems to be incorrect thou:

delX = (11.342885 - 11.318177) / (1813 - 247) = 0.024708 / 1566 = 1.577778 ( <- WRONG?)
delY = (50.983069 - 50.971017) / (1522 - 329) = 0.012052 / 1193 = 1.010226  ( <- WRONG?)

I need to have correct lat0/long0 values AS WELL AS correct delX/delY values in order to use imarshad's equation to convert Lat/Long to X/Y.

Keep going  - we're almost done here! ;)

-Jessica
Commented:
X= (lat0-lat)/delX
Y= (long-long0)/delY"
assumes that lat is aligned with the X axis and long is aligned with the Y axis, which if the three points you've supplied are accurate, is not true in your example.
Lat = a*x + b*y + c
Long = d*x + e*y + f
is more general, working for any orientation, although it still assumes an affine transform, which could be a bad assumption over a large area or near the poles.  But you seem to be working with a small enough area sufficiently far from the poles that an affine transform is probably good enough and you won't need anyting more complicated
Commented:
going from Lat,Long to x,y can be done by the same principle as going from x,y to Lat,Long

x = A*lat + B*long + C
y = D*lat + E*long + F
where
A = ((Long1-Long2)*(x2-x3)-(Long2-Long3)*(x1-x2))/((Lat2-Lat3)*(Long1-Long2)-(Lat1-Lat2)*(Long2-Long3));
B = ((Lat1-Lat2)*(x2-x3)-(Lat2-Lat3)*(x1-x2))/((Long2-Long3)*(Lat1-Lat2)-(Long1-Long2)*(Lat2-Lat3));
C = x1 - (a*Lat1 + b*Long1);

D = ((Long1-Long2)*(y2-y3)-(Long2-Long3)*(y1-y2))/((Lat2-Lat3)*(Long1-Long2)-(Lat1-Lat2)*(Long2-Long3));
E = ((Lat1-Lat2)*(y2-y3)-(Lat2-Lat3)*(y1-y2))/((Long2-Long3)*(Lat1-Lat2)-(Long1-Long2)*(Lat2-Lat3));
F = y1 - (d*Lat1 + e*Long1);

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Author Commented:
Thanks very much! That's perfect!
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