Saturday, October 15, 2005

Benchmarks, NOAA, more on Datum

Logged my first entry at Groundspeak with a 1111 feet high vertical control mark (corrected to 1115.59 feet) at Old San Jose Road. Also tried to figure out why Caltrans placed several marks around the Summit area on Hwy 17.

I didn't look much further to find out about those marks. They are quite easy to spot, just look for some orange stacks with "survey mark" on it. My guess is that they are probably keeping track of land movement due to earthquakes.


Mentor Software published a newsletter called "The Casual Cartographer" a while ago. In the June 98 edition it explained really well "What's a datum?" There are quite a few good articles under GIS Tips on Coordinate Systems that include great explanations on longitudes and latitudes.

The North American Datum of 1927 (NAD 27) was based in a single point in US, or a single datum. More specifically Meade's Ranch in Kansas (but the real "mother of all points" was established in 1879 in Maryland and it is named Principio, latin for begining). From that known location geodesists "used telescopes, pendulums, gravity meters [...] to determine the latitude and longitude of a single point, and the true azimuth to another point. This combination is called, in the geodetic sense, a datum. [...] By triangulation, given two known points you can measure and calculate adittional 'known points' using traditional surveying techniques."


If you read about the history of the Geodetic Surveys done by the National Oceanic and Atmospheric Administration (NOAA) you will come across some fascinating facts.

Like how the US Topographic maps were made, Alaska surveyed during the war, techniques and equipment used like the theodolite, angle measurements, how the UTM Grid was developed, the use of microwave and lasers with EDMI (Electronic Distance Measurement Instrument) and the men behind these works.


Surveying is probably one of the oldest techniques devised by man: keeping track of land. James Anderson in his "Surveying, Theory and Practice" defines it as "the art of measuring shape and horizontal and vertical distances between objects, of measuring angles between lines, of determining the direction of lines, and of establishing point locations a predetermined angular and linear measurements". He also differs between geodetic and plane surveying, where the mean surface of earth is considered a plane instead of a geoid.


There are quite a few good books on GPS published in recent years to follow in the footsteps of newer features and products. But along these titles you might come across a fine volume: Mapping by David Greehood [ISBN 0226306976] published by the University of Chicago Press. Original edition published in 1944, revised in 1964.

For example it sheds some light in why exactly we use A.M. (Ante Meridian) and P.M. (Post Meridian) to describe when the Sun has crossed the noon line or a prime meridian in a given location.

GPS became operational in the early 80's but despite its age you will find in a single volume with really neat graphics in a two column layout pretty much anything you need to know about orienting yourself on Earth, plus map making and the necessary concepts and tools. Highly recommended.

GIS again

After a brief stint with ArcGis I run across MapInfo's website and a favorable review of their software made in April 2005. Sounds like they figured out that easy-of-use is something that customers actually like. They do offer a free download for their software for evaluation.

Another option is the Pro version of Google Earth (formerly Keyhole) which offers import capabilities and let you create overlays by adding drawings, annotations and locations over a satellite photo.

GPS Toolkit

Looking at an year old issue of Linux Journal that Josh dropped at my cube the other day there was a review of GPStk, a C++ library sponsored by the University of Texas at Austin. You can download it from their Sourceforge repository and follow the directions to build either under plain Unix or with MS Visual 2005 in Windows. You will also need Jam a build tool that replaces Make.