I have a GPS shield that I got a couple of years ago from friends as a gift. The shield was tucked away in my parts drawer waiting for the right time to be used in a project. I'm not sure why I decided to build this project, because a hand held GPS is really quite useless these days. Maybe because I had all the components and I wanted to put the GPS module to use. The design is basic and revolves around four components: a GPS module, a Raspberry Pi, an 1.8" TFT LCD screen, and some push buttons for control. The goal is to build a stand alone, hand held, GPS device that will display its location on a moving map. The device works by using swaths of Google map screen shots stored on the SD card, along with an XML file that describes the map's boundary coordinates. GPS readouts are used to pick the proper map swath, and center a portion of the map to be displayed on the LCD. The map is also rotated to match the GPS heading readout. The device is battery powered with (maybe) the aid of solar cells to help charge the batteries during daylight. More on this project, including pics and video, here [https://sites.google.com/site/eyalabraham/gps]

+-------------------------------------------+
|                                           |
|               main.c                      |
|                 |                         |
|                 +-- test.c                |
|                 |                         |
|                 +-- nav.c                 |
|                      |                    |
|                      +-- util.c           |
|                                           |
+--------+------------+------+--------------+ 
|        |            |      |              |
|        | vt100lcd.c |      |              |
|        |            |      |  OS Serial   |
|        +------------+      |  driver      |
|                     |      |    +         |
|      pilcd.c        |      |  libxml2     |
|                     |      |              |
+---------------------+      |              |
|                            |              |
|        libbcm2835.o        |              |
|                            |              |
|                            |              |
+----------------------------+--------------+

See file descriptions below.

+------------+--------------+--------------+
|            :              :              |
|    SPI     :     GPIO     :     UART     |
|            :              :              |
+------------+--------------+--------------+
|            |              |              |
|     LCD    | Push buttons |     GPS      |
|            |              |              |
+------------+--------------+--------------+

• For GPIO and SPI using libbmc2835 [http://www.airspayce.com/mikem/bcm2835/] and its Python binding [https://github.com/mubeta06/py-libbcm2835]
• For UART serial communication with GPS module using Raspbian built in serial driver

Using Raspberry Pi image 2017-11-29-raspbian-stretch-lite.img with upgrades installed. Added libxml2 and libbcm2835 development packages. Installed and enabled SSH with public key and password-less logon, to allow rsync and rsh remote command execution. UART is enabled and SPI is disabled in raspi-config.

• main.c Main module
• test.c Contains various test routines activated by optional command line switch -t
• nav.c Main GPS and man navigation application.
• util.c Processing utilities, XML parsing, NMEA sentence parsing and coordinate conversions etc
• pilcd.c TFT LCD driver (ST7735 device) including text and graphics functions.
• vt100lcd.c VT100-aware prinf functions for LCD
• libbcm2835.so BCM2835 GPIO driver from [http://www.airspayce.com/mikem/bcm2835/index.html]
• mapcoord.py a Python test program that loads a map image and helped me validate the translation between GPS coordinates and pixels on an image of a map. This program uses OpenCV and a screen capture of a map. Due to the curvature of the earth, the map swaths need to be around 2 to 3 square miles. I did not test where the linearity of the mapping breaks.
• imgconvert.py This program takes in a map image in any format that OpenCV understand (screen capture) and its GPS coordinates, and converts the image to an LCD formatter RGB565 raw pixel data. The program updates a maps XML database file with the converted file's map attributes.
• showrawimg.py A program I use to validate the conversion done by imgconvert.py
• mapmeta.py A program I wrote to figure out how to use the XML Python library, which I ended up using in imgconvert.py and showrawimg.py
• nmea.py A parser for NMEA text sentences captured from GPS
• maproirotate.py A program I wrote to test the algorithms for extracting a rotated ROI from a larger image. The algorithm is implemented in nav.c function get_map_patch()
• logger2kml.awk AWK script that processes my GPS logger data and converts them to a KML file readable by Google Earth. The resulting KML can be used to display the logged paths on Google Earth.
• startup.sh A shell script used to auto start the navigation app in Raspberry Pi. Link through /etc/rc.local
• maps/ This directory holds the maps.xml description file that carries the necessary meta-data for the maps. The maps will also be stored here as raw RGP files converted by the imgconvert.py script from any graphic format into the necessary raw RGB. this directory should be copied onto a USB drive to be mounted on the Raspberry Pi file system.

The GPIO-wiring file lists the connections between the Raspberry Pi, the GPS Shield, LCD TFT, and pushbuttons. The rpi-gps-board.pdf shows the breadboard wiring.

       \|/
        |
        |
+-------+--------+
|                |
| GPS module     |
|                |
| ITEAD RoyalTek |
| REB-4216/      |
| REB-5216 GPS   |
|                |
+-------+--------+
        |
        | Serial UART
        |
+-------+--------+  +--------------+
|                |  |              |
|  Raspberry Pi  |  | LCD 1.8" TFT |
|                |  | ST7735R      |
|                |  |              |
+-------++-------+  +------++------+
        ||                 ||
========================================
                    SPI Bus

Not shown

• Push buttons for multi-function control (up/down/left/right/select) connected to Raspberry Pi's GPIO input lines.

• GPS module [https://www.itead.cc/wiki/Arduino_GPS_shield]
• Interpret NMEA sentences [http://www.gpsinformation.org/dale/nmea.htm] and [http://www.earthpoint.us/Convert.aspx]
• Extracting a rotated ROI, from Dr. Dobd's [http://www.drdobbs.com/architecture-and-design/fast-bitmap-rotation-and-scaling/184416337]
• Python XML [https://eli.thegreenplace.net/2012/03/15/processing-xml-in-python-with-elementtree]
• Using libxml2 [http://hamburgsteak.sandwich.net/writ/libxml2.txt]