Accessing a partitioned vector within a map

I have an input file which has the following structure

#Latitude   Longitude   Depth [m]   Bathy depth [m] CaCO3 [%] ...
-78 -177    0   693 1
-78 -173    0   573 2
.
.

I have created a map, which has a key based on a string (the name of an ocean basin) and a value which contains a vector of the data. Now I need to sort the vectors by bathyDepth. To be precise, I would like to partition the vectors so that I can have a partition between all data rows with a depth between 0 and 500m, 500m and 1500m, 1000m and 2000m...

I have stored the data into the map structure, but I'm unsure of how to store and access the partition so that I can then cout a data point at a particular depth.

My attempt:

//Define each basin spatially
//North Atlantic
double NAtlat1 = 0,  NAtlong1 = -70, NAtlat2 = 75, NAtlong2 = -15;
//South Atlantic and the rest...
double SPLIT = 0;

struct Point
{
   //structure Sample code/label--Lat--Long--SedimentDepth[m]--BathymetricDepth[m]--CaCO3[%]--CO3freefraction (SiO2 carb free)[%]--biogenic silica (bSiO2)[%]--Quartz[%]--CO3 ion[umol/l]--CO3critical[umol/l]--Delta CO3 ion[umol/kg]--Ref/source
   string dummy;
   double latitude, longitude, rockDepth, bathyDepth, CaCO3, fCaCO3, bSilica, Quartz, CO3i, CO3c, DCO3;
   string dummy2;
   //Use Overload>> operator
   friend istream& operator>>(istream& inputFile, Point& p);
};

//MAIN FUNCTION
std::map<std::string, std::vector<Point> > seamap;
seamap.insert( std::pair<std::string, std::vector<Point> > ("Nat", vector<Point>{}) );
seamap.insert( std::pair<std::string, std::vector<Point> > ("Sat", vector<Point>{}) );
//Repeat insert() for all other basins

Point p;
while (inputFile >> p && !inputFile.eof() )
{
    //Check if Southern Ocean
    if (p.latitude > Slat2)
    {
        //Check if Atlantic, Pacific, Indian...
        if (p.longitude >= NAtlong1 && p.longitude < SAtlong2 && p.latitude > SPLIT)
        {
            seamap["Nat"].push_back(p);
        } // Repeat for different basins
    }
    else
    {
        seamap["South"].push_back(p);
    }
}
//Partition basins by depth
for ( std::map<std::string, std::vector<Point> >::iterator it2 = seamap.begin(); it2 != seamap.end(); it2++ )
{
    for (int i = 500; i<=4500; i+=500 )
    {
        auto itp = std::partition( it2->second.begin(), it2->second.end(), [&i](const auto &a) {return a.bathyDepth < i;} );
    }
}

Note: a is of type Point. If I try to store itp into a structure such as a vector, I get the following error:

error: no matching function for call to ‘std::vector<Point>::push_back(__gnu_cxx::__normal_iterator<Point*, std::vector<Point> >&)’

I'm just unsure of how to store itp. The end goal is to calculate the distance between a data point and all the other data points within a particular depth window (e.g. 1500m to 2500m). Any help for this novice would be appreciated.