Upcoming Survey Scene newsletters will carry additional columns in this series.
Basic procedures and tools for determining valid published NAVD 88 GNSS-derived orthometric heights for constraints
These columns have provided the reader with basic concepts, routines and procedures for understanding, analyzing, evaluating and estimating GNSS-derived ellipsoid and orthometric heights.
In my last column, Part 7 (June 2016), we analyzed the changes in adjusted heights due to different leveling-derived NAVD 88 height constraints and compared the results with the published NAVD 88 leveling-derived orthometric heights. My column demonstrated how every constraint has an influence on the final set of adjusted heights.
As mentioned in previous columns, when incorporating new geodetic data into the National Spatial Reference System (NSRS), it is important to maintain consistency between neighboring stations. If the station has moved since the last time its height was established then not constraining the published value and superseding the height is the appropriate action to take. As I emphasized in Part 6 (April 2016), if the difference is not due to movement but due to some other reason such as the results of a previous adjustment distribution correction then superseding the height may not be the appropriate action to take. In Part 6, we looked at the network design of the NAVD 88 project and estimated the potential NAVD 88 distribution correction between two benchmarks involved in the original NAVD 88 general adjustment. It was also mentioned in the last newsletter that all of the analysis and recommendations have been based on using the latest scientific geoid model xGeoid15b.
However, in practice, GNSS-derived orthometric heights are incorporated into the NAVD 88 using the latest hybrid geoid model, i.e., GEOID12B. I recommend first performing the analysis using the scientific geoid model because the hybrid geoid model has been warped to be consistent with the published NAVD 88 values. This was described in detail in my October 2015 newsletter. The analysis using the scientific geoid should be included in the project report especially if the user finds significant differences between the results using the two different geoid models. In my last column, I stated that “maintaining consistency between closely spaced stations is extremely important when incorporating data into an existing network. Based on the information so far and the results using GEOID12B, I would not recommend constraining the published NAVD 88 heights of stations PHANIEL and PLAZA in the final NAVD 88 GNSS-derived orthometric height adjustment. These two stations resulted in significant changes in relative adjusted heights when they were constrained. (See Part 6.)”
It was also noted in a previous column (Part 5, February 2016) that 10 of the 2015 GNSS Rowan County Height Modernization project’s stations have published NAVD 88 GNSS-derived orthometric heights. These station are denoted as Height Modernization stations and are important because they are on the edge of the network where there’s a void of published NAVD 88 leveling-derived orthometric heights. In this newsletter, for these 10 stations we will look at the differences between their published NAVD 88 heights and their adjusted GNSS-derived orthometric heights from the Rowan County project.
First, we need to briefly look at one of the leveling-derived stations — Station PLAZA — that was identified as a potential outlier in Part 7. In that column, I provided the following information about station PLAZA:
|The geodetic data and information for station PLAZA is listed below:
Based on the available information to date, I would not recommend constraining the published height of station PLAZA in the final adjustment. Once again, this station’s published height should not be superseded by the GNSS project until new leveling has been performed between station FIFTH and PLAZA.
As I mentioned, Station PLAZA’s published height should not be superseded by the GNSS project until new leveling has been performed between station FIFTH and PLAZA. Well, ask and you will receive. Gary Thompson, the director of the North Carolina Geodetic Survey, had one of his field crews, which was in the area, relevel the section between station FIFTH and PLAZA. The newly leveled results changed the leveling-derived height of PLAZA relative to FIFTH by 3.5 cm. The new leveling-derived orthometric height of PLAZA now agrees with the GNSS-derived orthometric height to within a centimeter.
This means that the published height of PLAZA should not be constrained in the final adjustment and should be superseded by the GNSS-derived orthometric height. If the leveling data is submitted to NGS for inclusion into the NAVD 88, then the NAVD 88 height resulting from the new leveling data should be constrained in the final adjustment.
Now, let’s look at the 2015 GNSS Rowan County Height Modernization project’s stations that have published NAVD 88 GNSS-derived orthometric heights. The user can identify stations that have been established following NGS Height Modernization procedures by looking at NGS datasheets. The datasheets for Height Modernization stations have the following statement at the top of the datasheet: “This is a Height Modernization Survey Station.” In addition to that statement, the NAVD 88 orthometric height is published to the centimeter level with the attribute code of “GPS OBS.” (See the example titled “Excerpt from the NGS Datasheet for Station GOODMAN.)
Excerpt from the NGS Datasheet for Station GOODMAN
1 National Geodetic Survey, Retrieval Date = JULY 2, 2016
The procedures for analyzing the published NAVD 88 GNSS-derived orthometric heights are the same as those used to analyze the NAVD 88 leveling-derived orthometric heights. These procedures and routines have been documented in my previous columns. There is, however, one major difference between incorporating new leveling data into NAVD 88 and incorporating new GNSS data into NAVD 88. That is, when a station gets superseded in a leveling network adjustment due to previous adjustment distribution corrections, to maintain consistency the older leveling data in the area are readjusted to be consistent with the newly observed leveling data and latest published adjusted heights.
An adjustment distribution correction from the NAVD 88 general adjustment was discussed in the Part 7 (See Figure 6, “An Example of an Estimate of the NAVD 88 Distribution Correction Between two Stations Established with Old Leveling Data and Large Loops.”). So, what’s the difference?
Both NAVD88 leveling-derived orthometric heights and GNSS-derived orthometric heights are based on adjustments constraining NAVD 88 published orthometric heights. However, GNSS-derived orthometric heights are also computed using the latest NGS hybrid geoid model. If a station’s GNSS-derived orthometric height gets superseded, the previous GNSS data are not readjusted to be consistent with the latest observations and published heights. Once again, if the station physically moved then superseding the height is the appropriate action and there is no requirement to readjust the older GNSS data.
However, if the station did not physically move then the new published height may be inconsistent with its neighboring stations. I’m not saying that this is right or wrong, I’m only mentioning it so the user considers this information in their analysis.
The procedures outlined in NGS’ NGS 59 document, which was discussed in Part 5, were developed to minimize the effect due to different geoid models and superseded heights. (See excerpt titled “Four Basic Control Requirements for Estimating GNSS-Derived Orthometric Heights.”) The requirements include surrounding the project with valid NAVD 88 benchmarks and, if necessary, enlarging the project area to occupy enough leveling-derived benchmarks. The intent of these requirements are to help control any small relative differences between previously published hybrid geoid models. It should be noted that some of the latest hybrid geoid models are significantly different the older hybrid geoid models.
Therefore, when comparing a project’s adjusted heights with published NAVD 88 GNSS-derived orthometric heights, the user needs to consider which hybrid geoid model was used to establish the published GNSS-derived orthometric height. The NGS datasheet provides the hybrid geoid model and geoid height value used to establish the height. This was highlighted on the datasheet for station GOODMAN (see the example titled “Excerpt From the NGS Datasheet for Station GOODMAN). The statement NAVD 88 orthometric height was determined with geoid model GEOID09 means that station GOODMAN’s GNSS-derived orthometric height was established in a GNSS project using the hybrid geoid model GEOID09. The question is, what’s the difference between GEOID09 and the latest hybrid model?
The datasheet provides the hybrid geoid model value used to establish the height (in this example, GEOID09 = -30.377 m) as well as the latest hybrid geoid model value (in this example, GEOID12B = -30.402 m). Based on station GOODMAN’s published datasheet, the difference is only 2.5 cm. This difference may be much larger in the mountains of North Carolina.
Four Basic Control Requirements
Requirement 1: GNSS-occupy stations with valid NAVD 88 orthometric heights; stations should be evenly distributed throughout project.
Requirement 2: For project areas less than 20 km on a side, surround project with valid NAVD 88 benchmarks, i.e., minimum number of stations is four; one in each corner of project. [NOTE: The user may have to enlarge the project area to occupy enough benchmarks, even if the project area extends beyond the original area of interest.]
Requirement 3: For project areas greater than 20 km on a side, keep distances between valid GNSS-occupied NAVD 88 benchmarks to less than 20 km.
Requirement 4: For projects located in mountainous regions, occupy valid benchmarks at the base and summit of mountains, even if the distance is less than 20 km.
Station BLACK BEAR, located in the mountains near Asheville, North Carolina, is an example of a significant difference between GEOID09 and GEOID12B; the difference is -14.9 cm. (See the example titled “Excerpt from the NGS Datasheet for Station BLACK BEAR.) This may not be a problem if all stations in the area are effected by the same difference but that’s not the case in this area.
Station BUCK is a nearby station (about 11 km away from BLACK BEAR) and according to the NGS database “mark_source option”, stations BLACK BEAR and BUCK were involved in the same GNSS project so their GNSS-derived orthometric heights most likely were established in the same adjustment project. [NOTE: The use of the “mark_source” option of the NGS datasheet was described in Part 7.] The GEOID09 and GEOID12B difference at station BUCK is 1.0 cm. The relative difference in hybrid geoid models between stations BLACK BEAR and BUCK is almost 16 cm.
Excerpt from the NGS Datasheet for Station BLACK BEAR
PROGRAM = datasheet95, VERSION = 8.9
Figure 1 is a contour plot of the differences between GEOID12A and GEOID09 in the area surrounding stations BLACK BEAR and BUCK. [NOTE: The ESRI raster plots are based on GEOID12A not GEOID12B. GEOID12A is identical to GEOID12B everywhere, except in Puerto Rico and Virgin Island region. Therefore, in North Carolina, GEOID12A is equivalent to GEOID12B.] Looking at the plot it is obvious that there is a significant difference between the two hybrid geoid models in this region of North Carolina. What does this mean to someone performing a new GNSS-derived orthometric height adjustment in the area? If they occupied station BLACK BEAR and compared their adjusted GNSS-derived orthometric height using GEOID12B to the NAVD 88 published GNSS-derived orthometric height that was established using GEOID09, they most likely will get a large residual due to the difference between the two hybrid geoid models. As previously mentioned in this newsletter, NGS’ NGS 59 guidelines were developed to minimize the effects of different hybrid geoid models, but in these extreme cases the procedures may not have been able to minimize the total effect. It is important for the user to understand the differences between the various published hybrid models and experimental geoid models being developed by NGS. This topic was discussed in detail in the October 2015 newsletter.
Now, let’s look at the published NAVD 88 GNSS-derived orthometric heights occupied in the Rowan County Height Modernization project. Table 1 is a list of the stations occupied in the Rowan County project that have published NAVD 88 GNSS-derived orthometric heights. The table provides the hybrid geoid model value used to establish the published NAVD 88 height as well as the latest hybrid geoid model value, GEOID12B. Figure 2 is a contour plot of the differences between the GEOID12A and GEOID09 in the Rowan County Height Modernization project area. Looking at the plot, the user can see that most of the differences are all less than 3 cm between GEOID12A and GEOID09 in the Rowan County Project area.
As we can see from Table 1, all of the differences between the two hybrid geoid models are less than or equal to 2.5 cm. (See highlighted rows and column in Table 1.)
Figure 2 plots the adjusted GNSS-derived orthometric height (using GEOID12B) from a minimally constrained adjustment minus the published NAVD 88 GNSS-derived orthometric heights. Most of the differences are less than 3 cm which for some stations could be a result of the difference hybrid geoid models to establish the published GNSS-derived orthometric heights.
Looking at figure 2, almost all of the differences between the GNSS-derived orthometric heights (using GEOID12B) from the minimum-constraint least squares compared with the published NAVD 88 GNSS-derived orthometric heights are less than 3 cm. No station appears to be an obvious outlier. The fact that all differences except for one are negative is interesting and is worth investigating at a later date. More analysis will need to be performed to understand if this is significant or not. Table 2 provides the adjusted GNSS-derived heights from a minimally constrained adjustment minus the published heights (both ellipsoid and orthometric).
The last item to look at is a comparison of the adjusted heights from a constrained adjustment where all valid published leveling-derived heights were constrained. Figure 3 and Table 2 provide the constrained adjustment results (where all of leveling-derived published heights except for the 3 suspect heights were constrained) compared with the published NAVD 88 GNSS-derived orthometric heights. All of the differences are less than +/- 2 cm except for station NATHAN which is -2.1 cm. All of the relative differences of closely-spaced stations are less than 2 cm and most are less than 1 cm. This means constraining these stations should not adversely influence the unconstrained stations. Note that after constraining the published NAVD 88 leveling-derived heights, the negative bias is gone but the differences do not appear to be random. That is, the northern stations are all negative and the southern stations are positive (See figure 3).
These newsletters have focused on procedures and routines for establishing GNSS-derived orthometric heights. There are many ways to analyze and investigate GNSS data and adjustment results. I have provided some basic concepts that I believe are important for users to understand. The selection of constraints is a very important part of establishing accurate and consistent NAVD 88 GNSS-derived orthometric heights. It is just as important to document all decisions and results so others know how the published heights were established. NGS has a prescribed set of data and information that are required when submitted data for inclusion into the NSRS. This information is available from the NGS website (see section titled “MATERIALS NEEDED TO SUBMIT FOR THE PROJECT” in the document “adjustment_guidelines.pdf.”). We will address submitting the results in future columns.
In my next column, I will focus on the NGS GPS on BMS (GPSBM) dataset. This is the dataset used to create the hybrid geoid models; I mentioned this in Part 3. As mentioned in Part 3, the hybrid geoid model is designed to fit the published NAVD 88 leveling-derived orthometric heights. This file can be used to identify potential issues in the NAVD 88 network. GNSS users should be familiar with this dataset and how it can be useful to their analysis. My next column will address this topic.