Hello
Ultimate goal:
compute the distance between two points for differential GPS measurements (using Adafruit GPS module).
How I'm going about it:
I am receiving a long variable type (the other GPS) via a radio transmitter. The primary unit receives it's position directly from the attached GPS in the form of a double (I believe). I am multiplying that float by 10000 and attempting to cast it as a long. From there, I am calculating the difference and dividing the result by a decimal. For future math operations, I am using the abs() function to get the absolute value. From there, I am casting the value as a float because I am raising the value to the power of two using the function pow(). Finally, I am taking the square root of that (and another value with the same operations done to it) to get the distance.
However, I am getting occasional overflow issues and very odd numbers (generally very high). Thus, I am hoping someone can point out where the source(s) of error is(are). For help in the overflow area, I'm dealing with numbers around 37000000 and 121000000.
EDIT #1: I have just realized upon writing this that I do not need to take the absolute value before raising the number to the power of two, but I have decided to leave it in to ensure that I do not create a coding hole that doesn't make sense.
EDIT #2: Thank you everyone for your responses and help. We ended up using the Haversine distance formula (incorporates the Earth's radius). Coding for that proved to be much easier, and we realized that 1) doing math functions inside abs() isn't recommended, and 2) if the data type is a float, use fabs(). In the end, the project was successful, and we were able to achieve an error of about 10 feet using differential GPS calculation.
Thank you very much for your time and patience!
-Bob
Here is the code:
#define sec_feet_lat_conversion 986//.72753
#define sec_feet_lon_conversion 1537//.06506
struct roverRemoteData receivedData;//this is the data structure for the radio receiver
uint8_t rcvdSize = sizeof(receivedData);//this is for the size of the data structure
double parent_latitude;//GPS lat. position from GPS module for primary(parent) device
double parent_longitude;//GPS lon. position from GPS module for primary device
long child_latitude;//whole number received from secondary device via radio receiver
long child_longitude;//whole number received from....
long GPS_P_Latitude;
long GPS_P_Longitude;
double latitude_feet;
double longitude_feet;
double Latitude_Distance;
double Longitude_Distance;
double straight_line_distance;
parent_latitude = GPS.latitude;
parent_longitude = GPS.longitude;
child_latitude = receivedData.GPS_CLatitude;//data from radio receiver via structure
child_longitude = receivedData.GPS_CLongitude;//data from radio receiver via structure
GPS_P_Latitude = (long)(parent_latitude * 10000);//multiplying to get a whole number
GPS_P_Longitude = (long)(parent_longitude * 10000);
long c_p_lat_diff = (GPS_P_Latitude - child_latitude)/sec_feet_lat_conversion;//calc. difference and converting seconds to feet)
latitude_feet = abs(c_p_lat_diff);//ensuring positive value
float float_lat_feet = (float)latitude_feet;//casting as float for pow()
long c_p_lon_diff = (GPS_P_Longitude - child_longitude)/sec_feet_lon_conversion;
longitude_feet = abs(c_p_lon_diff);
float float_lon_feet = (float)longitude_feet;
Latitude_Distance = pow(float_lat_feet, 2.0);
Longitude_Distance = pow(float_lon_feet, 2.0);
straight_line_distance = sqrt(Latitude_Distance + Longitude_Distance);
