I've gone ahead and solved my problems here. What I did was create an ObjectiveC implementation of a c implementation of fortran code to do what I needed. The original c can be found here: http://sourceforge.net/projects/egm96-f477-c/
You would need to download the project from source forge in order to access the input files required for this code: CORCOEF and EGM96
My objective-c implementation is as follows:
GeoidCalculator.h
#import <Foundation/Foundation.h>
@interface GeoidCalculator : NSObject
+ (GeoidCalculator *)instance;
-(double) getHeightFromLat:(double)lat andLon:(double)lon;
-(double) getCurrentHeightOffset;
-(void) updatePositionWithLatitude:(double)lat andLongitude:(double)lon;
@end
GeoidCalculator.m
#import "GeoidCalculator.h"
#import <stdio.h>
#import <math.h>
#define l_value (65341)
#define _361 (361)
@implementation GeoidCalculator
static int nmax;
static double currentHeight;
static double cc[l_value+ 1], cs[l_value+ 1], hc[l_value+ 1], hs[l_value+ 1],
p[l_value+ 1], sinml[_361+ 1], cosml[_361+ 1], rleg[_361+ 1];
+ (GeoidCalculator *)instance {
static GeoidCalculator *_instance = nil;
@synchronized (self) {
if (_instance == nil) {
_instance = [[self alloc] init];
init_arrays();
currentHeight = -9999;
}
}
return _instance;
}
- (double)getHeightFromLat:(double)lat andLon:(double)lon {
[self updatePositionWithLatitude:lat andLongitude:lon];
return [self getCurrentHeightOffset];
}
- (double)getCurrentHeightOffset {
return currentHeight;
}
- (void)updatePositionWithLatitude:(double)lat andLongitude:(double)lon {
const double rad = 180 / M_PI;
double flat, flon, u;
flat = lat; flon = lon;
/*compute the geocentric latitude,geocentric radius,normal gravity*/
u = undulation(flat / rad, flon / rad, nmax, nmax + 1);
/*u is the geoid undulation from the egm96 potential coefficient model
including the height anomaly to geoid undulation correction term
and a correction term to have the undulations refer to the
wgs84 ellipsoid. the geoid undulation unit is meters.*/
currentHeight = u;
}
double hundu(unsigned nmax, double p[l_value+ 1],
double hc[l_value+ 1], double hs[l_value+ 1],
double sinml[_361+ 1], double cosml[_361+ 1], double gr, double re,
double cc[l_value+ 1], double cs[l_value+ 1]) {/*constants for wgs84(g873);gm in units of m**3/s**2*/
const double gm = .3986004418e15, ae = 6378137.;
double arn, ar, ac, a, b, sum, sumc, sum2, tempc, temp;
int k, n, m;
ar = ae / re;
arn = ar;
ac = a = b = 0;
k = 3;
for (n = 2; n <= nmax; n++) {
arn *= ar;
k++;
sum = p[k] * hc[k];
sumc = p[k] * cc[k];
sum2 = 0;
for (m = 1; m <= n; m++) {
k++;
tempc = cc[k] * cosml[m] + cs[k] * sinml[m];
temp = hc[k] * cosml[m] + hs[k] * sinml[m];
sumc += p[k] * tempc;
sum += p[k] * temp;
}
ac += sumc;
a += sum * arn;
}
ac += cc[1] + p[2] * cc[2] + p[3] * (cc[3] * cosml[1] + cs[3] * sinml[1]);
/*add haco=ac/100 to convert height anomaly on the ellipsoid to the undulation
add -0.53m to make undulation refer to the wgs84 ellipsoid.*/
return a * gm / (gr * re) + ac / 100 - .53;
}
void dscml(double rlon, unsigned nmax, double sinml[_361+ 1], double cosml[_361+ 1]) {
double a, b;
int m;
a = sin(rlon);
b = cos(rlon);
sinml[1] = a;
cosml[1] = b;
sinml[2] = 2 * b * a;
cosml[2] = 2 * b * b - 1;
for (m = 3; m <= nmax; m++) {
sinml[m] = 2 * b * sinml[m - 1] - sinml[m - 2];
cosml[m] = 2 * b * cosml[m - 1] - cosml[m - 2];
}
}
void dhcsin(unsigned nmax, double hc[l_value+ 1], double hs[l_value+ 1]) {
// potential coefficient file
//f_12 = fopen("EGM96", "rb");
NSString* path2 = [[NSBundle mainBundle] pathForResource:@"EGM96" ofType:@""];
FILE* f_12 = fopen(path2.UTF8String, "rb");
if (f_12 == NULL) {
NSLog([path2 stringByAppendingString:@" not found"]);
}
int n, m;
double j2, j4, j6, j8, j10, c, s, ec, es;
/*the even degree zonal coefficients given below were computed for the
wgs84(g873) system of constants and are identical to those values
used in the NIMA gridding procedure. computed using subroutine
grs written by N.K. PAVLIS*/
j2 = 0.108262982131e-2;
j4 = -.237091120053e-05;
j6 = 0.608346498882e-8;
j8 = -0.142681087920e-10;
j10 = 0.121439275882e-13;
m = ((nmax + 1) * (nmax + 2)) / 2;
for (n = 1; n <= m; n++)hc[n] = hs[n] = 0;
while (6 == fscanf(f_12, "%i %i %lf %lf %lf %lf", &n, &m, &c, &s, &ec, &es)) {
if (n > nmax)continue;
n = (n * (n + 1)) / 2 + m + 1;
hc[n] = c;
hs[n] = s;
}
hc[4] += j2 / sqrt(5);
hc[11] += j4 / 3;
hc[22] += j6 / sqrt(13);
hc[37] += j8 / sqrt(17);
hc[56] += j10 / sqrt(21);
fclose(f_12);
}
void legfdn(unsigned m, double theta, double rleg[_361+ 1], unsigned nmx)
/*this subroutine computes all normalized legendre function
in "rleg". order is always
m, and colatitude is always theta (radians). maximum deg
is nmx. all calculations in double precision.
ir must be set to zero before the first call to this sub.
the dimensions of arrays rleg must be at least equal to nmx+1.
Original programmer :Oscar L. Colombo, Dept. of Geodetic Science
the Ohio State University, August 1980
ineiev: I removed the derivatives, for they are never computed here*/
{
static double drts[1301], dirt[1301], cothet, sithet, rlnn[_361+ 1];
static int ir;
int nmx1 = nmx + 1, nmx2p = 2 * nmx + 1, m1 = m + 1, m2 = m + 2, m3 = m + 3, n, n1, n2;
if (!ir) {
ir = 1;
for (n = 1; n <= nmx2p; n++) {
drts[n] = sqrt(n);
dirt[n] = 1 / drts[n];
}
}
cothet = cos(theta);
sithet = sin(theta);
/*compute the legendre functions*/
rlnn[1] = 1;
rlnn[2] = sithet * drts[3];
for (n1 = 3; n1 <= m1; n1++) {
n = n1 - 1;
n2 = 2 * n;
rlnn[n1] = drts[n2 + 1] * dirt[n2] * sithet * rlnn[n];
}
switch (m) {
case 1:
rleg[2] = rlnn[2];
rleg[3] = drts[5] * cothet * rleg[2];
break;
case 0:
rleg[1] = 1;
rleg[2] = cothet * drts[3];
break;
}
rleg[m1] = rlnn[m1];
if (m2 <= nmx1) {
rleg[m2] = drts[m1 * 2 + 1] * cothet * rleg[m1];
if (m3 <= nmx1)
for (n1 = m3; n1 <= nmx1; n1++) {
n = n1 - 1;
if ((!m && n < 2) || (m == 1 && n < 3))continue;
n2 = 2 * n;
rleg[n1] = drts[n2 + 1] * dirt[n + m] * dirt[n - m] *
(drts[n2 - 1] * cothet * rleg[n1 - 1] - drts[n + m - 1] * drts[n - m - 1] * dirt[n2 - 3] * rleg[n1 - 2]);
}
}
}
void radgra(double lat, double lon, double *rlat, double *gr, double *re)
/*this subroutine computes geocentric distance to the point,
the geocentric latitude,and
an approximate value of normal gravity at the point based
the constants of the wgs84(g873) system are used*/
{
const double a = 6378137., e2 = .00669437999013, geqt = 9.7803253359, k = .00193185265246;
double n, t1 = sin(lat) * sin(lat), t2, x, y, z;
n = a / sqrt(1 - e2 * t1);
t2 = n * cos(lat);
x = t2 * cos(lon);
y = t2 * sin(lon);
z = (n * (1 - e2)) * sin(lat);
*re = sqrt(x * x + y * y + z * z);/*compute the geocentric radius*/
*rlat = atan(z / sqrt(x * x + y * y));/*compute the geocentric latitude*/
*gr = geqt * (1 + k * t1) / sqrt(1 - e2 * t1);/*compute normal gravity:units are m/sec**2*/
}
double undulation(double lat, double lon, int nmax, int k) {
double rlat, gr, re;
int i, j, m;
radgra(lat, lon, &rlat, &gr, &re);
rlat = M_PI / 2 - rlat;
for (j = 1; j <= k; j++) {
m = j - 1;
legfdn(m, rlat, rleg, nmax);
for (i = j; i <= k; i++)p[(i - 1) * i / 2 + m + 1] = rleg[i];
}
dscml(lon, nmax, sinml, cosml);
return hundu(nmax, p, hc, hs, sinml, cosml, gr, re, cc, cs);
}
void init_arrays(void) {
int ig, i, n, m;
double t1, t2;
NSString* path1 = [[NSBundle mainBundle] pathForResource:@"CORCOEF" ofType:@""];
//correction coefficient file: modified with 'sed -e"s/D/e/g"' to be read with fscanf
FILE* f_1 = fopen([path1 cStringUsingEncoding:1], "rb");
if (f_1 == NULL) {
NSLog([path1 stringByAppendingString:@" not found"]);
}
nmax = 360;
for (i = 1; i <= l_value; i++)cc[i] = cs[i] = 0;
while (4 == fscanf(f_1, "%i %i %lg %lg", &n, &m, &t1, &t2)) {
ig = (n * (n + 1)) / 2 + m + 1;
cc[ig] = t1;
cs[ig] = t2;
}
/*the correction coefficients are now read in*/
/*the potential coefficients are now read in and the reference
even degree zonal harmonic coefficients removed to degree 6*/
dhcsin(nmax, hc, hs);
fclose(f_1);
}
@end
I've done some limited testing against the Geoid Height Calculator (http://www.unavco.org/community_science/science-support/geoid/geoid.html) and looks like everything is a match
UPDATE iOS8 or Greater
As of IOS8 This code might not work correctly. You may need to change how the bundle is loaded:
[[NSBundle mainBundle] pathForResource:@"EGM96" ofType:@""];
Do some googling or add a comment here.
