Leaderboard

NGS has developed a new beta tool for obtaining geodetic information about a passive mark in their database. This column will highlight some features (available as of Oct. 5, 2020) that may be of interest to GNSS users. It provides all of the information about a station in a more user-friendly format. The box titled “Passive Mark Lookup Tool” is an example of the webtool. The tool provides a lot of information so I have separated the output of the tool into several boxes titled “Passive Mark Lookup Tool — A through D.” I will highlight several attributes that I believe will be very useful to users, especially users of leveling-derived and GNSS-derived orthometric heights. I’ve highlighted several attributes in the box titled “Passive Mark Lookup Tool — A” that are important to users such as published coordinates, their datum and source, Geoid18 value, GNSS Useable, and the date of last recovery. All of these values are available on a NGS datasheet but, in my opinion, this provides the information in a more user-friendly format. One calculation that the user can easily compute for marks that have been leveled to and occupied by GNSS equipment, is the difference between the published leveling-derived orthometric height and the computed GNSS-derived orthometric height. This may indicate that the mark has moved since the last time it was leveled to or that its height coordinate has been readjusted since the creation of the published geoid model. The table below provides the calculation using the data from the box titled “Passive Mark Lookup Tool — A.” The calculation [HGNSS = hGNSS — NGeoid18; Difference = HGNSS — HNAVD 88] has been described in several of my previous columns In this example, the difference between the GNSS-derived orthometric height and the Published NAVD 88 height is 6.1 cm. NGS is looking for comments on this beta webtool so if users would like this computation added to the tool, they should send a comment to NGS using the link provided on the site (This is a beta product. NGS is interested in your feedback concerning its function and usability as well as how users would like to interact with NGS datasheet information in the future. Email us at [email protected].) So, the user should ask the question, did the station move since the last time it was leveled? Another attribute that would be nice to be part of this tool is was the station used to create the hybrid geoid model. As of Oct. 5, 2020, users have to go to the Geoid18 webpage to get the information. The excel file and shapefiles provides whether the station was used to create the Geoid18 model. In the case of this example, KK1531, CHAMBERS, the mark was not used in the creation of Geoid18 so NGS felt that the station may have moved and/or the GPS on Bench Mark residual was large relative to its neighbors. See NGS’s technical report on Geoid18 for more information on the creation of Geoid18. The GPS on Bench Mark residual analysis was described in several of my previous columns (see “The differences between Geoid18 values and NAD 83, NAVD 88 values” and “NGS 2018 GPS on BMs program in support of NAPGD2022 — Part 6” for examples). The webtool provides a map depicting the location of the station, photos (if available), and previously published, superseded values of the mark. See the box titled “Passive Mark Lookup Tool — B.” https://www.gpsworld.com/wp-content/uploads/2020/10/zilkoski-beta-tool-column-image-2.jpg source: https://www.gpsworld.com/ngs-releases-beta-tool-for-obtaining-geodetic-information/

Earth is known as the “Blue Planet” due to the vast bodies of water that cover its surface. With an over 70% of our planet’s surface covered by water, ocean depths offer basins with an abundance of features, such as underwater plateaus, valleys, mountains and trenches. The average depth of the oceans and seas surrounding the continents is around 3,500 meters and parts deeper than 200 meters are called "deep sea". This visualization reveals Earth’s rich bathymetry, by featuring the ETOPO1 1-Arc Minute Global Relief Model. ETOPO1 integrates land topography and ocean bathymetry and provides complete global coverage between -90° to 90° in latitude and -180° to 180° in longitude. The visualization simulates an incremental drop of 10 meters of the water’s level on Earth’s surface. As time progresses and the oceans drain, it becomes evident that underwater mountain ranges are bigger in size and trenches are deeper in comparison to those on dry land. While water drains quickly closer to continents, it drains slowly in our planet’s deepest trenches. These trenches start to become apparent below 5,000 meters, as the majority of the oceans have been drained of water. In the Atlantic Ocean, there are two trenches that stand out. In the southern hemisphere, the South Sandwich trench is located between South America and Antarctica, while in the northern hemisphere the Puerto Rico trench in the eastern Caribbean is its deepest part. The majority of the world’s deepest trenches though are located in the Pacific Ocean. In the southern hemisphere, the Peru-Chile or Atacama trench is located off the coast of Peru and the Tonga Trench in the south-west Pacific Ocean between New Zealand and Tonga. In the northern hemisphere, the Philippines Trench is located east of the Philippines, and in the northwest Pacific Ocean we can see a range of trenches starting from the north, such as the Kuril-Kamchatka, and moving to the south all the way to Mariana’s trench that drains last. It is worth recalling that the altitude values of ETOPO1 range between 8,333 meters (topography) and -10,833 meters (bathymetry). This range of altitude values reflects the limitations of the visualization, since Challenger Deep - the Earth’s deepest point located at Mariana's trench - has been measured to a maximum depth of 10,910 meters and Mount Everest the highest peak above mean sea level is at 8,848 meters. In this visualization the vertically exaggerated by 60x ETOPO1 relief model, utilizes a gray-brown divergent colormap to separate the bathymetry from topography. The bathymetry is mapped to brownish hues (tan/shallow to brown/deep) and the dry land to greys (dark gray/low to white/high). A natural consequence of this mapping is that areas of the highest altitude are mapped to whitish hues, as they are almost always covered in snow. Furthermore, in an effort to help viewer’s eyes detect surface details that would otherwise be unnoticeable, the topography and bathymetry have been rendered with ambient occlusion - a shadowing technique that in this particular visualization darkens features and regions that present changes in altitude, such as mountains, ocean crevices and trenches. download: https://svs.gsfc.nasa.gov/vis/a000000/a004800/a004823/OceanDrain_Colorbar_1920x1080_30fps.mp4 https://svs.gsfc.nasa.gov/vis/a000000/a004800/a004823/OceanDrain_1920x1080_30fps.mp4 source: https://svs.gsfc.nasa.gov/4823