Modifying the NDBI values using MODIS bands data

import matlab.io.hdfeos.*

import matlab.io.hdf4.*

for i=1

direc=strcat('F:\Bands_data\Terra\');

cd(direc)

files = dir(fullfile( '*.hdf'));

for j=1:size(files,1)

filename=files(j).name;

FILE_NAME=strcat('F:\Bands_data\Terra\',filename);

GRID_NAME='MOD_Grid_500m_Surface_Reflectance';

DATAFIELD_NAME='sur_refl_b01';

file_id = gd.open(FILE_NAME, 'rdonly');

grid_id = gd.attach(file_id, GRID_NAME);

data = gd.readField(grid_id, DATAFIELD_NAME, [], [], []);

[xdimsize, ydimsize, upleft,lowright] = gd.gridInfo(grid_id);

gd.detach(grid_id);

gd.close(file_id);

data=double(data);

data=data';

SD_id = sd.start(FILE_NAME, 'rdonly');

sds_index = sd.nameToIndex(SD_id, DATAFIELD_NAME);

sds_id = sd.select(SD_id, sds_index);

fillvalue_index = sd.findAttr(sds_id, '_FillValue');

fillvalue = sd.readAttr(sds_id, fillvalue_index);

long_name_index = sd.findAttr(sds_id, 'long_name');

long_name = sd.readAttr(sds_id, long_name_index);

units_index = sd.findAttr(sds_id, 'units');

units = sd.readAttr(sds_id, units_index);

scale_index = sd.findAttr(sds_id, 'scale_factor');

scale = sd.readAttr(sds_id, scale_index);

scale = double(scale);

offset_index = sd.findAttr(sds_id, 'add_offset');

offset = sd.readAttr(sds_id, offset_index);

range_index = sd.findAttr(sds_id, 'valid_range');

range = sd.readAttr(sds_id, range_index);

sd.endAccess(sds_id);

sd.close(SD_id);

data(data==fillvalue) = NaN;

data(data < range(1)) = NaN;

data(data > range(2)) = NaN;

data = (data - offset) * scale;

Band1(:,:,j)=data;

clear data

j

end

end

clearvars -except Band2 Band6 Band1

%%

b1=mean(Band1,3,'omitNaN');

b2=mean(Band2,3,'omitNaN');

b6=mean(Band6,3,'omitNaN');

b7=mean(Band7,3,'omitNaN');

%%

NDBI=(double(b6)-double(b2))./(double(b6)+double(b2));

ndbire=griddata(double(lat),double(lon),NDBI,h24v6lat,h24v6lon);

reshapeAOD=reshape(MeanAOD,1697000,1);

reshapeNDBI=reshape(ndbire,1697000,1);

reshapeddatandbi=cat(2,reshapeAOD, reshapeNDBI);

reshapeddatandbi(any(isnan(reshapeddatandbi),2),:)=[];

correlation_NDBI = corr(reshapeddatandbi(:,1),reshapeddatandbi(:,2));

This is the code used to generate Normalized Difference Built-Up Index. In the output figure the lower left portion is water area and the water area is also classified into built up area and built up area is lying in negative values range. I need to figure this out where am i doing wrong?