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Adding bilinear interpolation algorithm for elevation determination #52

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Adding bilinear interpolation algorithm for elevation determination #52

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3b67691
Fixing misleading javadoc comment.
Oct 9, 2019
ffd9730
Updating interpElevationAt() to utilize the lower points in the inter…
Oct 9, 2019
aeaf6ec
Adding bi-linear interpolation algorithm for elevation post values.
Oct 9, 2019
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117 changes: 112 additions & 5 deletions src/openmap/com/bbn/openmap/dataAccess/dted/DTEDFrame.java
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Original file line number Diff line number Diff line change
Expand Up @@ -187,8 +187,7 @@ protected boolean reopen() {
// ////////////////

/**
* The elevation at the closest SW post to the given lat/lon. This is just a
* go-to-the-closest-post solution.
* The elevation at the closest post to the given lat/lon.
*
* @param lat latitude in decimal degrees.
* @param lon longitude in decimal degrees.
Expand All @@ -211,7 +210,60 @@ public int elevationAt(float lat, float lon) {
}
return -32767; // Considered a null elevation value
}

/**
* Bi-linear interpolated elevation at a given lat/lon - should be more
* precise than elevationAt(), but that depends on the resolution of the
* data.
*
* @param lat latitude in decimal degrees.
* @param lon longitude in decimal degrees.
* @return elevation at lat/lon in meters.
*/
public int biLinearInterpElevationAt(float lat, float lon) {
if (frame_is_valid == true) {
if (lat >= dsi.sw_lat &&
lat <= dsi.ne_lat &&
lon >= dsi.sw_lon &&
lon <= dsi.ne_lon) {

// lat/lon_post_intervals are *10 too big -
// extra 0 in 36000 to counteract
float lat_index = (lat - dsi.sw_lat) * 36000F / uhl.lat_post_interval;
float lon_index = (lon - dsi.sw_lon) * 36000F / uhl.lon_post_interval;

int lflon_index = (int) Math.floor(lon_index);
int lclon_index = (int) Math.ceil(lon_index);
int lflat_index = (int) Math.floor(lat_index);
int lclat_index = (int) Math.ceil(lat_index);

if (elevations[lflon_index] == null)
readDataRecord(lflon_index);
if (elevations[lclon_index] == null)
readDataRecord(lclon_index);

int ul = elevations[lflon_index][lclat_index];
int ur = elevations[lclon_index][lclat_index];
int ll = elevations[lflon_index][lflat_index];
int lr = elevations[lclon_index][lflat_index];

float answer = biLinearInterpolation(
ul,
ur,
lr,
ll,
lflon_index,
lflat_index,
lclon_index,
lclat_index,
lat_index,
lon_index);
return Math.round(answer);
}
}
return -32767; // Considered a null elevation value
}

/**
* Interpolated elevation at a given lat/lon - should be more precise than
* elevationAt(), but that depends on the resolution of the data.
Expand All @@ -231,7 +283,7 @@ public int interpElevationAt(float lat, float lon) {

int lflon_index = (int) Math.floor(lon_index);
int lclon_index = (int) Math.ceil(lon_index);
/* int lflat_index = (int) Math.floor(lat_index); */
int lflat_index = (int) Math.floor(lat_index);
int lclat_index = (int) Math.ceil(lat_index);

if (elevations[lflon_index] == null)
Expand All @@ -257,8 +309,8 @@ public int interpElevationAt(float lat, float lon) {

int ul = elevations[lflon_index][lclat_index];
int ur = elevations[lclon_index][lclat_index];
int ll = elevations[lflon_index][lclat_index];
int lr = elevations[lclon_index][lclat_index];
int ll = elevations[lflon_index][lflat_index];
int lr = elevations[lclon_index][lflat_index];

float answer = resolveFourPoints(ul, ur, lr, ll, lat_index, lon_index);
return Math.round(answer);
Expand Down Expand Up @@ -393,6 +445,61 @@ public short[][] getElevations(int startx, int starty, int endx, int endy) {
// Internal methods
// ////////////////

/**
* Perform a bilinear interpolation of 4 known elevation post values.
*
* @param ul elevation value for the upper left coordinate.
* @param ur elevation value for the upper right coordinate.
* @param lr elevation value for the lower right coordinate.
* @param ll elevation value for the lower left coordinate.
* @param flon lower (floor) longitude index
* @param flat lower (floor) latitude index
* @param clon upper (ceiling) longitude index
* @param clat upper (ceiling) latitude index
* @param lat_index Target latitude location.
* @param lon_index Target longitude location.
* @return A bi-linear interpolation of the input known values.
*/
private float biLinearInterpolation(
int ul,
int ur,
int lr,
int ll,
int flon,
int flat,
int clon,
int clat,
float lat_index,
float lon_index) {

float R1 = 0.0f;
float R2 = 0.0f;
float result = 0.0f;

if (((clon - lon_index) == 0.0f) &&
((lon_index - flon) == 0.0f)) {
R1 = ll;
}
else {
R1 = (clon - lon_index) * ll + (lon_index - flon) * lr;
}
if (((clon - lon_index) == 0.0f) &&
((lon_index - flon) == 0.0f)) {
R2 = ul;
}
else {
R2 = (clon - lon_index) * ul + (lon_index - flon) * ur;
}
if (((clat - lat_index) == 0.0f) &&
((lat_index - flat) == 0.0f)) {
result = (R1 + R2)/2.0f;
}
else {
result = (clat - lat_index) * R1 + (lat_index - flat) * R2;
}
return result;
}

/**
* A try at interpolating the corners of the surrounding posts, given a lat
* lon. Called from a function where the data for the lon has been read in.
Expand Down