<div dir="ltr"><div><div><div><div><div><div><div><div><div>Hi Michal<br><br></div>After looking at the source code of proj4 what I'm conclude is (and without 100% certainty) :<br></div><br>1) difference on lla file are on micro_sec. Because on nad2bin.c the input read from lla file are converted from micro_sec to radian. see ligne 137 of nad2bin.c : <br>
"t.lam = (laml += lam) * U_SEC_TO_RAD;"<br></div><br>2) concerning the lla format and after reading the part "Reading the ASCII Table" of nad2bin.c I'm agreed with your conclusion about <ddx> ( as before it is was I understand from lignes 132-140 of nad2bin.c)<br>
<br></div>3) The nadgrid file should be on Local LatLong. If you look at pj_transform.c when there is/are nadgrids files: for the conversion from local (source) latlong to wgs84 latlong there is a call to <br> pj_apply_gridshift_2( srcdefn, 0, point_count, point_offset, x, y, z );<br>
</div>and from wgs84 to destination (local lat/Long) there is a call to <br> pj_apply_gridshift_2( dstdefn, 1, point_count, point_offset, x, y, z );<br></div>the second parameter is "inverse" so the first call use the direct way and the second call the inverse way. <br>
<br></div>4) the "inverse" is computed with iteration (do while loop) , you could see that from the nad_cvt.c file. <br><br></div>5) for geoidgrids the inverse is only a "minus" operation because the input and output coordinate are on the same planimetric system. **The question** is what is this planimetric system. see some discussion on lastools list (<a href="https://groups.google.com/forum/#!topic/lastools/jRV0bvyAlJQ">https://groups.google.com/forum/#!topic/lastools/jRV0bvyAlJQ</a>)<br>
<br></div>Hope that could help you.<br><div><div><div><div><div><div> <br></div><div>Nicolas David<br></div></div></div></div></div></div></div><div class="gmail_extra"><br><br><div class="gmail_quote">2014-04-20 21:15 GMT+02:00 Michal Seidl <span dir="ltr"><<a href="mailto:michal.seidl@gmail.com" target="_blank">michal.seidl@gmail.com</a>></span>:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hello,<br>
1) I spent some time testing and studying proj code and this is my<br>
conclusion about *.lla format that is used by nad2bin<br>
<br>
The data are organized:<br>
<line number>: <dx> <dy> <ddx> <ddy> .....<br>
<br>
<ddx> means difference between current dx and dx at previous position(left).<br>
So to get dx at some position needs get dx from begging of line plus sum of<br>
ddx.<br>
<br>
Can someone confirm or explain?<br>
<br>
2) As I am playing with nadgrids another question rise up. There is just one<br>
nadgrids file. In what coordinate system it should be? In WGS84 LatLong or<br>
Local LatLong? If you go from local system to WGS84 how can you interpolate<br>
in gird tha is in WGS84 system?<br>
a) some iteration or other clever algorithm<br>
b) do not care, correction should not change so dramatically<br>
<br>
Probably the same question applies to geoidgrids?<br>
<br>
<br>
Best regards Michal<br>
<br>
<br>
<br>
--<br>
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