.swift

let addFloodButton = UIButton(frame: CGRect.zero)
        addFloodButton.setImage(UIImage(named: "plus-image"), for: .normal)

        addFloodButton.addTarget(self, action: #selector(addFloodAnnotation), for: .touchUpInside)

        addFloodButton.translatesAutoresizingMaskIntoConstraints = false
        
        self.view.addSubview(addFloodButton)
        
        addFloodButton.centerXAnchor.constraint(equalTo: self.view.centerXAnchor).isActive = true
        addFloodButton.bottomAnchor.constraint(equalTo: self.view.bottomAnchor, constant: -40).isActive = true
        addFloodButton.widthAnchor.constraint(equalToConstant: 75).isActive = true
        addFloodButton.heightAnchor.constraint(equalToConstant: 75).isActive = true