Forum virtualnega letalstva Slovenije

sm nekaj mi povejte.. kk naj pretvorim tiste bedne koordinate od nvatlasa da bojo primerne za FSa .. sm iskal one transformacijske zadeve sm majo neko omejitev pa to...

Hi

For most FS purposes I assume the earth as one sphere and I use this equality:

1 nautical mile = 1852 meters

Following are some notes derived on this simple assumption.

Distance between 2 points at the same longitude
=====================================

If 2 points P2 and P1 differ one MINUTE of latitude, the distance
along the meridian arc, in meters, will be:

DY = 1852

If 2 points P2 and P1 differ one DEGREE of latitude, the distance
along the meridian arc, in meters, will be:

DY = 1852 * 60 = 111120

If P2 is at latitude Lat2 and P1 is at latitude Lat1, the distance
along the meridian arc, in meters, will be:

DY = 111120 * ( Lat2 - Lat1 )

Distance between 2 points at the same latitude
====================================

At the equator one MINUTE of longitude is 1852 meters but, at the poles, it
will be zero. Assuming a sphere and 2 points, P2 and P1, at latitude equal
to Lat and longitudes equal to Lon2 and to Lon1 respectively, the distance
between the points along the parallel arc is:

DX = 111120 * ( Lon2 - Lon1 ) * cos( Lat)

Useful SCASM macro coding
====================

In order to evaluate delta_SN and delta_WE distances in RefPoint units between 2 points of known latitudes and longitudes, the following code can be used:

; Macro Parameters:
; 1 = latitude
; 2 = longitude
; 3 = aux latitude
; 4 = aux longitude
; 5 = scale

Uvar( $Lat0 %1 )
Uvar( $Lon0 %2 )

Uvar( $Lat1 %3 )
Uvar( $Lon1 %4 )

Uvar( $Scale %5 )

UVar( $Lat < [$Lat0 + $Lat1> / 2 ] )

Uvar( $DX <111120 / $Scale * cos($Lat) * [$Lon1 - $Lon0> ] )
Uvar( $DY <111120 / $Scale * [$Lat1 - $Lat0> ] )


Another View to the "SDK LOD table"
===========================

In the SDK LOD table, a LOD_0 quadrant (not a square!) is said to span 10018863 meters. This value is the "span along a meridian arc" and it is different from the "span along the equator".

I get a different number using the above formulae. Since a LOD_0 spans 90 degrees of latitude, that translates to meters:

90 * 60 * 1852 = 10000800

Since a LOD_0 spans 120 degrees of longitude the "span along the equator" is:

120 * 60 * 1852 = 13334400

For other quadrants we can construct this table:

http://www.ptsim.com/downloads/sdk_lod.htm

Aerial photos and custom textures
=========================

We can create photographic scenery from aerial photos very easily using the MS "resample tool". We only need to pass to the "resample tool" the coordinates of the NW corner and the coordinates of the SE corner of the master bitmap. The tool will resample the bitmap and will produce 256 x 256 bitmaps which will fill LOD_13 quadrants.

It has been said that better image quality can be achieved if the master bitmap is already supplied to the resample tool in its final resolution. If so the resample tool just divides the master in pieces without any resizing or scaling. At the equator one LOD_13 measures 1221 x 1628 meters. At latitude of 41:24:00 it measures approximatelly 1221 x 1221 meters. It is worth noting the LOD_13 size at different latitudes:

Lat. Size
00 1221 x 1628
10 1221 x 1603
20 1221 x 1530
30 1221 x 1410
40 1221 x 1247
50 1221 x 1046
60 1221 x 814
70 1221 x 557


If we have orto-rectified aerial photos we have to distort them if we want zero (or minimum) resizing by the "resample tool". At the equator one LOD_13 spans a rectangle of 1221 x 1628 meters so that the photograph should be compressed horizontally by 75% (1221/1628).

This pre-distortion in photographs is also very important when we try to match a photo with a mesh. In this case we can take a snapshoot of the mesh in TMFViewer and use it as a template to position the photograph. In order to get good results the photographs should be distorted horizontally (amplified at latitudes greater than 41 degrees and compressed near the equator).

Kind Regards, Luis