My 2c on the matter: 1) GPS (Global Positioning System) is a specific set of satellites, from the US. But there's also GLONASS, from Russia, and others are planned for the future, such as GALILEO (Europe) and BeiDou-2/COMPASS (China). The general term for "satellite-based positioning method" is GNSS (Global Navigation Satellite System). The device you buy at a store is a GPS receiver (if it only works with the US GPS system), or a GNSS receiver, if it can access more than one system. I'm not being pedantic, there is practical relevance to this (below).
2) A good rule of thumb is average accuracies of ~15-30m for phones, tablets and simple handheld receivers, ~10-15m for better handheld receivers with good antennas. These use the so called coarse-acquisition code, at the L1 frequency. GNSS receivers that can combine data from more than one system (GPS + GLONASS) are very widespread now, and they will improve these estimates simply by having access to more satellites at any given time. If you need sub 10-meter average accuracy, you'll need "survey grade" receivers that use the L2 carrier frequency (precision code). L2 receiver prices can vary by 2 orders of magnitude depending on several factors. And they usually need an external antenna, which costs extra. If you need sub-meter precision, then you need to use differential GPS (DGPS), where the surveying receiver is corrected by a second, stationary receiver (base receiver) that will quantify the expected error during your survey. There are some subscription services that allow you to use data from their own network of base stations, instead of your own second receiver, such as OmniStar (http://www.omnistar.com/). But your receiver must support these technologies. Many governments also have public base stations whose data can be accessed, but they might not have bases near you area of interest. The most important though, is to realize that "differential GPS" means having more than one receiver (to compare differences). I've seen several people bummed from not getting the expected accuracies from their "differential GPS", when they actually have only a single receiver. DGPS is a method, that requires specific receivers for using it, but is not a type of device per se. By the way, RTK (Real Time Kinematic) is a type of DGPS positioning where the correction is done in real time (as opposed to after the survey, a.k.a post-processing). 3) A cheap way to improve the accuracy of a mobile device is to use an external GPS, connected to the phone/tablet (or even a PC) via USB or bluetooth. These units tend to have external/better antennas, which are important for better accuracy. 4) There has been an explosion in sub-US$1k DGPS receiver chips in the market now, mostly aimed at the drone market, but they are transitioning into consumer products as well: http://store.swiftnav.com/s.nl/it.A/id.4734/.f . These are bare chips, that could be could be hooked up to a RasperryPi or Arduino if you're into a DYI solution.Might be an interesting project for an engineering undergrad. But these chips are enabling cheaper consumer products as well: http://bad-elf.com/pages/be-gps-3300-detail (I have no affiliation or previous experience with this specific product). Cheers, -- *Thiago Sanna Freire Silva* Professor Assistente Doutor Ecosystem Dynamics Observatory <https://sites.google.com/site/ecosystemobservation/> Departamento de Geografia Instituto de Geociências e Ciência Exatas (IGCE) Universidade Estadual Paulista (UNESP) Rio Claro, SP - Brasil Google Scholar Citations <http://scholar.google.ca/citations?user=hpPTEmcAAAAJ> Currículo Lattes <http://buscatextual.cnpq.br/buscatextual/visualizacv.do?id=P315543> On Mon, Apr 4, 2016 at 7:24 PM, Merav Vonshak <merav...@gmail.com> wrote: > Hi all, > I would greatly appreciate your input on a handheld GPS device. I’m > looking for a GPS device that would allow me to navigate to my field sites, > including uploading plot locations from Google Earth or ArcGIS and viewing > them in the field, over an aerial photo of the site. I will also need to > record data points. I’m considering using an iPhone 6 or a Google Nexus 9 > device, wondering about the pros and cons of each options and the app I > should use in case I choose the iPhone or the Nexus. > Thank you! > Merav > >
