Use struct data type to declare, create and use it in a function. The struct functions are abstracted in a seperate folder to show how it can be used and referred in the main.ino.
The struct LatLong can be declared:
struct LatLong {
  double latitude;
  double longitude;
};
The struct can be defined:
LatLong latlong = {1.2480631, 103.8285657};
The struct can be used in a function declaration:
void goToSentosa(struct LatLong *);
The struct can be used in a function definition:
void goToSentosa(struct LatLong *ll) {
  ...
}
#include "src/latlong/latlong.h"
LatLong latlong = {1.2480631, 103.8285657};
void setup() {
  Serial.begin(9600);
  while (!Serial) { }
  Serial.println("Locations of places around the world!");
}
void loop() {
  goToSentosa(&latlong);
  Serial.print("Sentosa: (");
  Serial.print(latlong.latitude, 6);
  Serial.print(", ");
  Serial.print(latlong.longitude, 6);
  Serial.println(")");
  goToEiffelTower(&latlong);
  Serial.print("Eiffel Tower: (");
  Serial.print(latlong.latitude, 6);
  Serial.print(", ");
  Serial.print(latlong.longitude, 6);
  Serial.println(")");
  delay(2000);
}BOARD?=esp32:esp32:t-beam
PORT?=/dev/cu.SLAB_USBtoUART
.PHONY: default lint all flash clean
default: all flash clean
lint:
	cpplint --extensions=ino --filter=-legal/copyright *.ino
all:
	arduino-cli compile --fqbn $(BOARD) ./
flash:
	arduino-cli upload -p $(PORT) --fqbn $(BOARD)
clean:
	rm -r build