The sound of her tears echoed as they cascaded into the serene waters below, declaring, “I will not let them kill my people!”
As I ascended Cerro de la Reina (Queen’s Hill) in Tonala, Mexico, camera in hand, thoughts swirled about the bullshit societal constructs that disempower women in modern North America. In our Latin America histories, and in many cultures worldwide, women have been revered as la madre tierra sagrada, or sacred mother earth goddesses. Our ancestral Latino narratives are adorned with tales of Latina princesses, queens, and warriors.
Finally, I reached the top of the hill, the site of a powerful indigenous Queen.
Her name was Cihualpilli Tzapotzintli (pronounced zoo-ah-pee-yee), a warrior-queen who oversaw a mini-empire of towns and cities. She had access to many resources from 12 tributary states. Cihualpilli surveyed the valleys of her lands from the observatory on top of the hill. All was relatively peaceful until the year 1530.
A view of the valleys below from on top of the Queen’s Hill.
A wave of Spanish conquistadors were closing in. Messengers alerted Cihualpilli of the menacing, armor-plated men. Tales of prior conquests haunted her, where defiance led to brutal slaughter and unimaginable atrocities. All who opposed the metal-men died a horrific death. Any survivors died from the pure evil spirits* of such men (*diseases that were brought into the area).
Aware of the inevitable clash, Cihualpilli opted for diplomacy, dispatching emissaries bearing gifts to meet the approaching conquistadors. However, the greed of the invaders demanded submission to the Spanish crown.
Cihualpilli had a policy to not attack the men. But dissidents from nearby tribes started to ally, worrying that the Queen could not lead in such war matters. She was waiting to welcome them with open arms, but the dissidents hid around the base of the hill, showering the metal-men with a rain of arrows. Within 2 hours, all the dissidents were dead.
She cried at a waterfall, “I will not let them kill my people!” Desperate to avert further bloodshed, Cihualpilli devised an unconventional strategy. Adorned in their finest attire, groups of women approached the invaders, offering lavish feasts, even themselves, in a bid to seduce and placate the conquerors. This ploy seemed effective, as Cihualpilli sought to surrender to the Spanish, envisioning a path to peaceful coexistence.
Submitting herself as an example, Cihualpilli underwent baptism by the conquistadors, adopting the name Juana Bautista Danza while retaining her symbolic role as ruler of Tonala.
The indigenous temple, once a site of sun worship, fell to the Christianization wave, its stones repurposed for a Christian sanctuary. Sneaking my camera inside, I captured remnants of the past.
Inside the Christian temple built from recycled stones. -akThe temple as it appears today. It is still active for sermons.
Today, statues of the Christianized Cihualpilli stand beside the Christian temple, while an indigenous depiction graces the town square, a testament to a complex history of adaptation and resilience.
A statue of Cihualpilli before becoming Christianized stands tall in the heart of Tonala’s city.A statue of Cihualpilli after becoming Christianized stands next to the temple. -ak
Nestled in the midst of the Turkana Basin, the world’s fourth largest salt lake rests: Lake Turkana. The regions around the lake has produced numerous key hominin fossils that shape the understanding of our human origins.
Conventional methods of creating topographic maps have produced maps of differing qualities: some with remarkable accuracies, some with poor accuracies. The maps may exhibit different scales and resolutions that are inconsistent with other maps created by conventional methods. The factor of coverage due to logistic, economic, and/or political restraints contribute to missing maps across the earth. The issue of such inconsistent (conventional) topographic maps can be remedied by using an earth-wide consistent mapping strategy. The 1990s saw the rise of synthetic aperture radar (SAR) interferometry, heightening the debate of creating a consistent, affordable, and efficient global digital elevation model (DEM).
NASA’s Shuttle Radar Topography Mission: The Road to Consistency.
NASA set out to obtain the first Earth-wide high-resolution elevation topographic database of our planet using SAR interferometry. NASA’s mission–called the Shuttle Radar Topography Mission (SRTM)–repurposed and modified radar instruments from prior Space Shuttle missions for its own goals. A second antenna was added to the SRTM. A series of C-band and X-band transmit and receive antennas were retrofitted in the Space Shuttle Endeavour. The strategy using two or more SARs to capture surface deformation or digital elevation is referred to as Interferometric Synthetic Aperture Radar (InSAR). The differences of radar transmitted waves hitting the Earth’s surface were collected using two SRTM antennas and recorded on highspeed tape systems. This resulted in capturing topographic data similar to that of stereo photography. The reader is referred to the cited literature to understand the complexities of this technology.
A space-artist’s conception of the SRTM. The green radar waves are transmitted to the Earth’s surface while the red waves represent the reflected data being transmitted back to the shuttle. Image credit: Detlev Van Ravenswaay http://www.vanravenswaay.com/index.aspx
On February 11, 2000, The Endeavour Space Shuttle launched into orbit. The Endeavour Space Shuttle orbited the Earth 176 times over a period of 11 days before returning to its home on February 22, 2000. Due to missed orbits, the SRTM did not capture some regions in North America and Antarctica. The SRTM was the first space-borne technology to capture a near-global (80% of the Earth) topographical database. The data was publicly made available in 2014.
A personal research account was created, granting access on USGS EarthExplorer. Using the interactive interface, the position of the Earth was oriented over the Turkana Basin. Under search criteria, a defined area was drawn around basin (see Figure 1) using the map coordinate tool. With the research region selected, the Data Sets tab was open with the specific criteria to search and display SRTM 1 Arc-Second Global data only. The query displayed the available SRTM grids for my pre-defined region. The interactive USGS EarthExplorer allowed me to visually examine the loci of the SRTM grids on my research area using the Show Footprint feature. Using the Browse Overlay feature allowed a small preview of the raw SRTM data inside of a grid (see 5th grid in Figure 1).
Figure 1 – USGS EarthExplorer workflow.
After visually verifying SRTM grid coverage for my research region, each SRTM grid was downloaded in the Georeferenced Tagged Image File Format (GeoTIFF). GeoTIFF is a standard image format used in GIS applications worldwide. GeoTIFFs encodes MetaTags within the image that defines projection types, coordinates and coordinate systems, ellipsoids, and other data. The GeoTIFF files were stored locally on the laboratory computer at the Paleoenvironmental Research Laboratory at Rutgers University for DEM processing and a raw copy archived on the cloud (Kuilan, 2018). A list of the SRTM GeoTIFF files used for this research are found at the end of this blog post.
SRTM – Accuracies & Known Limitations
Although a primary goal of the SRTM was to collect data that was globally consistent, producing consistent data with quantified known errors were part of the mission. An extensive ground-truth effort using kinematic Global Positioning System (KGPS) transects was undertaken by NASA. The data were collected by land vehicles equipped with Global Positioning System (GPS) receivers along roads that can be identified by radar. The KGPS spanned six continents and the data collection effort harvested almost 9.4 million samples. An additional 86,774 Ground Control Points (GCPs) were used from NASA affiliates. The collective efforts show that ground truthing the SRTM data has an accuracy of roughly 50 cm.
The SRTM team wanted to achieve an absolute vertical accuracy within 16 m with 90% confidence. The vertical accuracies vary around the Earth, increasing in high altitude regions (e.g. Tibetan Plateau, Mount Everest). SRTM has been independently tested by various researchers since SRTM data is being widely used in various applications. Multiple studies found that SRTM’s goal of vertical accuracy and confidence are met, but one independent study found that data in some continents did not adhere to SRTM’s goal. However, the cumulative studies show that the continent of Africa—where our research site is found—adheres to SRTM’s goal. Africa has a vertical accuracy of ~10 m, more than SRTM’s goal of 16 m.
I’d
like to note that a possible limitation of using SRTM data is that it will not
account for topographical changes since its recording. The SRTM was a one-time
mission that captured topographical data in 2000. Any significant or gradual
land changes that occurred post-2000 due to weathering, erosion, tectonic
activities, deposition, or other geological processes will not be reflected in
contemporary SRTM terrain analyses.
The Software: Global Mapper
Global Mapper is the software used for the interpolation of SRTM data to construct our DEM of Lake Turkana. The origins of Global Mapper extend back over two decades. It was developed by the United States Geological Survey (USGS) as a Microsoft Windows tool to view their products. The tool was known as DLGV32. The USGS released the source code into the public domain in 1998. Users continued to expand DLGV32. An individual created a commercial version of the tool—DLGV32 Pro—in 2001. Within that year, DLGV32 Pro evolved in “Global Mapper,” a product of Global Mapper Software LLC, while the USGS kept distributing the DLGV32 Pro version. In 2011, Blue Marble Geographics purchased Global Mapper LLC.
The interface of Global Mapper with the Turkana Basin DEM.
Blue Marble Geographics further evolved Global Mapper to be a well-respected software in the geographic information system (GIS) world. The software is a competitive leader against other popular GIS software—such as ArcGIS—due to its small footprint and affordability. Global Mapper is able to open a wide range of popular GIS, 3D, Gridded Elevation, Raster, and Vector formats. Global Mapper has numerous integrated features that include geocoding, digitizing, rendering, data processing, GPS tracking, spatial database support, graphing, LiDAR processing, and support for Unmanned Aerial Vehicle (UAV) data. It is my opinion that what makes Global Mapper powerful is not only its collaborative design team, but their focus on listening to the real-world GIS experiences and needs of scientists, researchers, and other users. This allows Global Mapper’s team to keep evolving their product, integrating new features in succeeding releases.
Methodology in Global Mapper
The version of Global Mapper used for the construction of the Turkana Basin DEM is v19.1 x64 edition with LiDAR module. The SRTM GeoTIFF files were opened within Global Mapper and saved as a Global Mapper Workspace (*.gmw). The Create Elevation Grid under the Analysis menu was selected and initiated. After a period of computer processing time, the Elevation Grid was completed. The processed SRTM data was now exported to DEM format.
The Enable Hill Shading feature was enabled. Shading uses light direction and shadows to emphasize the topography of a terrain, creating a 3D effect that visually allows a researcher to see how flat or hilly a region is. Global Mapper has options with dynamic variables to manipulate the direction of the light and the darkness of shadows. For our model, these options were left at default with light direction at altitude 45°, azimuth 45°.
Shaded relief is often colored.
It’s a method to represent topography in a natural, aesthetic, and intuitive
manner. There are many philosophies and techniques of using colors in
topography. For example, a researcher creating a topographic map of a dense
vegetated Amazonian region may want to use different shades of greens to
differentiate botanical life, or a terrain analyst may want to create a theme
of brown colors of their arid, mountainous site. The researcher can also artistically represent the topographic
regions with the colors that are pleasing to them. Global Mapper has a variety
of options to create custom color schemes, modify values of slope colors, or
import palette files of different formats. Global Mapper has 10 color presets
for shaded relief. For our model, no custom color or modifying of existing
color occurred. I used 3 pre-defined stock color presets to create 3 DEMs:
Atlas Shader, Global Shader, and Slope Shader.
Our color shaders result in hypsometric DEMs using the Atlas and Global shaders. It represents the elevation of the terrain with the applicable colors in gradient intervals. An Elevation Legend and Distance Scale were added to DEMs from Global Mapper’s Configuration tool. The Slope Shader based DEM has a Slope Degree legend. Legends and scales were set at 75% transparency.
3 high-resolution PDFs and 3 high-resolution JPGs were exported from Global Mapper using Atlas, Global, and Slope shaders (see Figures 1-4). Due to the rise of using Google Earth for geological applications, I decided to also export 3 high-resolution KMZ files. KMZs are Google’s Keyhole Markup Language (KML) file format that are compressed. KML/KMZ is a language that’s focused on geographic visualization and the annotation of its images and maps. The small-scale image depictions of the actual JPGs and PDFs on this article make it difficult for the viewer to appreciate the scale of detail contained. Figures 5 & 6 contains zoomed regions in an attempt for the reader to understand its scale of detail. (A list of all exported files are found at the end of this blog post.)
Figure 2 – Small scaled example of the exported Atlas Shader high-res JPG.
Figure 3 – Small scaled example of the exported Global Shader high-res JPG.
Figure 4 – Small scaled example of the exported Slope Shader high-res JPG.
Below are the exported KMZ files added to Google Earth Pro.
Image from Winder et al. 2013. The Complex Topography hypothesis, or “Scrambler Man”
A multitude of other research postulates the correlation of land features to hominid evolution events. One research puts forth the “Scrambler Man” hypothesis. The argument is that the use of rugged topography by early hominins to monitor and hunt mammals would have selected for transition to an upright posture, greater speed, and agility over time. The application of DEMs might be valuable to flesh out this hypothesis and possibly correlate applicable hominin features in other rugged landscapes.
It is my opinion that the noted DEM applications can be of value to the rich ongoing fieldwork at the Turkana Basin.
Summary and Future Directions
The collecting of SRTM data and interpolating it into DEMs of the Turkana Basin via Global Mapper was successful. Because of the long periods of processing and resulting file sizes, it would be ideal to interpolate SRTM data to DEM on smaller, unique regions that are of primary interest to the researcher. This will allow exporting at maximum resolutions while reducing file size footprints.
I wish to create DEMs from ASTER satellite data and compare it to the SRTM DEMs via layers in Global Mapper. Further exploiting the terrain analyses tools of Global Mapper will be of use for in-depth analysis of DEMs (see following images).
3D view in Global Mapper from DEM interpolated from SRTM Data 3D view with generated contour lines overlayed on the DEM A vertical elevation profile generated in Global Mapper using the line draw tool. The red line on top is a Cut-and-Fill feature designed to summarize the areas and volumes of change. This feature if used with satellite derived DEMS from different points in time, will allow the identification and analyses of surface material removed, surface material addtion, and areas that are unchanged Zoomed section of DEM terrain layered with generated contour linesWatershed analysis via Global MapperGenerated contours from DEM via Global MapperRandom 3D view of Turkana Basin TerrainTurkana Basin 3D view in Wiremode
