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POInconsistent result with VC++ versus Java
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copied!<p>I have a long-established routine written in Visual C++ running on PC which calculates the distance between two points on the earth's surface given their lat/long coordinates.</p> <p>I have converted this to Java as part of a port to Android, but I am getting different results. To investigate the reason for this, I have split a complex trig equation into bits to track what is happening at each step.</p> <p>However, in Visual C++ the result of the step by step calculation is different from the result obtained from the complex statement, although they should obviously be the same. I would like to know why!</p> <p>The Java versions yields the same result in both cases, as would be expected.</p> <p>Here is the Visual C++ code:</p> <pre><code>double CCoord::GetDistanceRadians(double dLatRadians, double dLongRadians) { double dResult; // This is the step by step code double r1, r2, r3, r4, r5; r1 = sin(m_dLatRadians); r2 = sin(dLatRadians); r3 = cos(dLatRadians); r4 = m_dLongRadians - dLongRadians; r5 = cos(r4); r1 *= r2; r2 = r3 * r3; r2 *= r5; r4 = r1 + r2; // This is the original statement: r4 and dResult should be equal // (but they aren't) dResult = sin(m_dLatRadians) * sin(dLatRadians) + cos(m_dLatRadians) * cos(dLatRadians) * cos(m_dLongRadians - dLongRadians); // Protect against maths processor inaccuracies if ((dResult > 1.0) || (dResult < -1.0)) dResult = 0.0; else dResult = acos(dResult); return dResult; } </code></pre> <p>And here is the Java code:</p> <pre><code> // Return angular distance in radians private double getDistanceRadians(double dLatRadians, double dLongRadians) { double result; // Do JIT test checkRadianValues(); // This is the step by step code double r1, r2, r3, r4, r5; r1 = Math.sin(m_dLatRadians); r2 = Math.sin(dLatRadians); r3 = Math.cos(dLatRadians); r4 = m_dLongRadians - dLongRadians; r5 = Math.cos(r4); r1 *= r2; r2 = r3 * r3; r2 *= r5; r4 = r1 + r2; // This is the original statement: r4 and result should be equal // (and they are) result = Math.sin(m_dLatRadians) * Math.sin(dLatRadians) + Math.cos(dLatRadians) * Math.cos(dLatRadians) * Math.cos(m_dLongRadians - dLongRadians); // Protect against maths processor inaccuracies if ((result > 1.0d) || (result < -1.0d)) result = 0.0d; else result = Math.acos(result); return result; } </code></pre> <p>These are the input and result values:</p> <pre><code>Variable VC++ value Java value m_dLatRadians 0.896808347 0.896808378 m_dLongRadians -0.047414778(0) -0.047414778(6) dLatRadians 0.896192633(9) 0.896192588(9) dLongRadians -0.026897463 -0.026897463 r4 1.000218106 1.0002181433 dResult/result 0.999917766 1.0002181433 </code></pre> <p>Please can anyone suggest why r4 and dResult should be different in the Visual C++ version?</p> <p>P.S. I get good final distance results from the VC++ version, and bad final results from Java - nearly all distances come out as zero (the higher level code which calls these routines is not shown because there is a lot of it! However it's the discrepancy I am interested in finding out about).</p>

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