Initial approach to evaluating available digital geo data

A previous post pointed out that some existing mapping of surficial deposits at 100k was inadequately detailed for the purposes of this project. We quickly made that assessment by draping the linework on imagery viewed in Google Earth and in ArcGIS. It is very likely that had the various geologists who developed this mapping over the years had such easy access to high-resolution imagery and GIS, then the mapping would be considerably less inadequate!


The example above shows the lines from the Lake Mead 100k map (USGS open-file data) in Google Earth. It works very well when zoomed into specific areas (down to 1:6000, for example), but the overall heterogeneity of the base imagery is a bit distracting. From this image (and the next) you can see that the detail in the bedrock is somewhat to considerably greater than that in the 'dirt'.

The image below is from ArcGIS and the lines from the Lake Mead 100k sheet are overlain on NAIP imagery. This imagery is also great when zoomed in and is homogeneous with respect to overall color balance, tone, etc. It is also from a much smaller window of time.


Our team has recently set up an image service at the UNR Geospatial Lab (Geography Dept.) that allows each investigator on the project to access the same high-resolution imagery remotely. Thus, we do not have multiple copies of giant .tiff files on all of our various computers. This is extremely helpful for collaboration and ensuring that everyone has equal access to the base imagery.

Surficial Geology Detail Comparison: Ivanpah Valley

There are two data sets of surficial geologic mapping that cover the Clark County part of Ivanpah Valley. The first one is from an NBMG map that I made. It is far too detailed to serve as a basis for mapping the entire county:


The same area from the 100k Mesquite Lake map shows considerably less detail, but relies on composite / combined units to account for this in most cases:


The Mesquite Lake snip above is from a map in which Ivanpah Valley is only a moderately small part. The map I developed (with help) was focused entirely on Ivanpah. As mentioned in a previous post, we are leaning toward some point between these two renditions. The NBMG map (published at 50k but mapped at ~12k) is excessively detailed and the USGS 100k map is a bit too general for what we would like to develop with the ND2MP. For example, we hope to map fewer composite units.

We suspect that we will ultimately end up closer to the USGS characterization of Ivanpah than to the NBMG characterization....not sure yet. We are actively applying generalization routines of various sorts to the NBMG data set. I will post a few examples next week.

An example of coarse detail on 100k map

As promised, and with no offense intended, here is an example of an area from an existing 100k geology dataset of Clark County that is clearly inadequately detailed from my perspective as a surficial mapper:
I made the contacts red to be obvious. Most of the tonal variations that you see in the image represent distinct surficial piedmont units. Many that have been lumped together are quite large and also span a huge range of time as far as surficial deposits go. Also it is not clear why some large active washes were mapped individually and other, larger ones weren't. This approach to mapping is covered in the unit descriptions from this map for the most part, but for our purposes, additional mapping is certainly required.

Digital Dirt Map Compilation Chronicles, Part 1 of MANY

To date, the ND2MP 'staff' has been compiling all of the existing, digital renditions of the geology of Clark County, Nevada. Luckily, the USGS has been compiling and developing 100k maps across the county and has, thus, done a fair amount of work for us already. What follows is a descriptive update of what we have been doing. Stay tuned for some graphic examples.

We have started to evaluate the relative worth of the various map data that we have in hand, and have gained some important insights.

First off, kudos to those authors who have pored over existing maps of a range of scales to develop their compilations (e.g., USGS versions of the Las Vegas 100k, Lake Mead 100k sheets). We are keenly aware of the huge amount of work that went into that process, and we are also aware that it involved new mapping in some areas. Even more kudos to those authors who generated large amounts of original mapping at similar scales (e.g. USGS version of the Mesquite Lake 100k). That was obviously a huge effort.

Given this, however, there are several facts about these maps that bear directly on our efforts. For example, there is a high degree of variability in the level of detail in the compilation maps. This fact is pointed out by the authors, so this comes as no shock to anyone. However, as we develop the surficial geologic map of the entire County, we are interested in developing a dataset that has a consistent level of detail across the entire area. This will be a large task. It will involve mainly enhancing the detail in existing compilations, but will also involve some generalization of overly detailed areas.

The latter point applies, for example, to the Ivanpah Valley area, where House et al., mapped in detail and Schmidt and McMackin mapped more generally. What we want for the ND2MP is somewhere in between those extremes, so we are experimenting with some automated generalization routines with the House et al. data and comparing the results to the Schmidt and McMackin mapping. I will post some examples when they are ready. Areas where the existing compilations are simply too general or appear somewhat arbitrary in detail will require significant amounts of new mapping as part of this project.

As for generalizing existing overly detailed maps, we are going to establish a minimum map unit criterion between 5 and 10 hectares. This refers to the areal extent below which we will not show a polygon. With respect to existing compilations, we plan to express polygons below the threshold as point-features in the database. We are also experimenting with ways to efficiently eliminate parts of polygons that are less than 30-50 meters wide. This issue arises mainly in the area of single-thread active washes. Ideally, we can collapse the narrow polys to centerlines that retain the atttribute when needed. This is sort of an 'annealing' function that digital cartographers are experimenting with, but we can't find any explicit add-in for doing it in ArcGIS except when it involves the distance between two polygons and not parts of the same polygon (doesn't create the line we want, however). When we progress in these areas, I will post examples to this blog.

Another arguably more important issue is that the existing compilations use different nomenclature for the surficial deposits. Given that several of these maps probably had overlapping compilation periods, are contiguous, and are from the same agency this is a bit surprising. However, I too have some pretty schizophrenic labeling schemes on my own maps and NBMG has no formal standard, so I can relate somewhat. In any case, for a county wide depiction of surficial geology that we want to ultimately apply statewide, it is absolutely necessary that we develop a consistent, flexible, and understandable framework. Each of the existing compilations provide some good and well-reasoned examples. We will begin with them and either choose the one we think is the best, or possibly confuse the issue more by developing a framework that we think is better. In any case, we will develop a rubric that explains how the various schemes relate. Currently, we are leaning toward a composite of the Las Vegas 100k approach with the Mesquite Lake 100k approach.

Stay tuned for an upcoming post that provides an opportunity to view and comment on our proposed scheme and its rationale. I will also prepare some examples of areas in need of generalization or more detailed mapping to support my statements above.

Ecosystem Indicators Project Research Objectives

ND2MP is supported in part by the Ecosystem Indicators (EIP), funded by the Clark County Multiple Species Habitat Conservation Program (MSHCP). The EIP has three main research objectives, ND2MP being a portion of one of those objectives. Objective one is to establish a robust GIS-based characterization of the geomorphology and surficial geology of Clark County, Nevada (i.e. ND2MP). Although the term ‘habitat’ is often used loosely as equivalent to ‘native vegetation’, this is not always the case. Nonetheless, scientist and managers continue to use vegetation as a means for defining habitat. Geologic materials provide an alternative and at times more appropriate means of defining species habitat and extent of vegetation communities, especially in arid environments (Miller and Franklin 2002, Heaton et al. 2006).

Objective two is to map the ecosystems in Clark County. The MSHCP uses an ecosystem-based approach to conservation planning and management for plant and animal species. We will update and refine the current vegetation based ecosystem model for Clark County. This includes the identification and spatial modeling of various NatureServe based Ecological Systems, Alliances and possible Associations. Our goal is to capture important ecosystems that include such broadly defined vegetation classes as blackbrush, sagebrush, pinion-juniper, saltbrush, creosote bush, Joshua Trees, etc. Additionally, we will for the first time develop spatial models for non-vegetation based MSHCP ecosystems such as dunes and dry lakes.

Objective three is to develop an enhanced vegetation based ecosystem classification model in three pilot areas using more intensive sampling, advanced spatial statistical methods and object-oriented classification and the newly developed geomorphology and surfical geology datasets. Current pilot areas under consideration include Piute-Eldorado Valleys, Ivanpah Valley, Gold-Butte and Kyle Canyon area across to the Sheep Range.