GMDE Step-By-Step.Pages

GMDE Step-byStep Guide

Accomplishing Tasks in GMDE: a step-by-step guide

GMDE is a powerful program for extracting quantitative data from a geologic map or satellite image. This document provides a step by step approach to accomplishing common tasks in the program as a supplement to the users manual.

There are many detailed adjustments, not covered here but reviewed in the Users manual, that one can make via the Inspector palette. Most important is perhaps the choice of elevation server that GMDE uses to get elevations of points. The default is the Map- Quest server which makes SRTM-based topo data available globally. If you are working with maps from the United States, you might want to switch to the USGS elevation server which uses the national elevation database and tends to be closer to the values you see on topo maps.

Contents

Georeference a Map 2 Set the Coordinate System Offset 3 Digitize Existing Strike and Dip Symbols 4 Calculate Orientations from 3-Point Problems 5 Select a Subset of Strikes and Dips for Analysis Using Lasso 7 Measure Stratigraphic Map Thicknesses from 3-Point Problem 8 Measure Stratigraphic Map Thicknesses using mapped strike and dip 9 Propagate Errors in Your Analysis 10 Digitize Contacts 11 Construct a Down-Plunge Fold Profile 12 Get the Elevation of a Cylindrically Folded Surface at Depth 14 Construct Topographic Profiles with Contacts and Apparent Dips 15 Plot Topo Profile Along Part of a Line of Section, only 18 Calculate Piercing Points on a Fault or other Planar Surface 18

Georeference a Map

Before a map can be used for quantitative calculations, you need to georeference the map. This will set the scale and enable the program to get the elevation of any clicked point automatically from the Internet. If you georeference a map, you do not have to set the scale separately! Once you have opened a scanned map, before doing anything else, proceed as follows.

1. Select Settings>Georeference (⌘G). This dialog box (to the right) will lead you through the entering of the four corners of the map, starting with the upper left corner.

2. In the map pane, click carefully on the coordinates at the upper left corner of the map. GMDE will automatically select the box where you will type in the longitude of the corner point. Then, you just need to tab to the latitude box. Note that west longitudes and south lati- tudes are entered with a negative number. All values are entered in decimal degrees.

3. Once you’ve click on the upper left corner of the map and entered the coordinates, GMDE is ready and waiting for you to click on the upper right corner. Once you do so, text boxes will appear where you can enter the longitude and latitude as before. GMDE attempts to save you some typing and assumes that the latitude will be the same as that previously entered, but you can change that to the actual value if they are different.

4. Proceed as above with the lower right corner and then the lower left corner. If you are georeferencing a standard map, GMDE will have correctly ﬁlled in the coordinates completely for the lower right corner

5. When all four points have been entered, click okay. GMDE will then ask you what units you want to use for the map. The georeferencing procedure generally works with maps and images in a variety of different projections because they are “close enough” to a UTM projection. In some cases where the map or satellite image is too distorted or in a geographic projection, GMDE will inform you that the least squares best ﬁt of the map to a UTM projection is poor and to proceed with caution. In that

case, you may want to use a different software package to reproject the map into a UTM projection to start with.

Set the Coordinate System Offset

If you plan on using GMDE’s ability to download elevations for clicked points from elevation servers on the Internet, it is especially important that you set the coordinate system offset. This offset is the difference between whatever datum was used for the map (and commonly you will not know because it is not listed on the map) and a WGS84 datum, which is what elevation servers use. GMDE can calculate a uniform offset for the entire map area by comparing the coordinates of a point in Google Earth with the same point on the scanned map you are working with. Proceed as follows:

1. Locate a point that can be easily and accurately identiﬁed both in Google Earth and on your map. Usually, this will be a road intersection, the intersection of two creeks or maybe a building or well deﬁned peak, meander bend, etc.

2. In Google Earth, determine the longitude and latitude of the feature and write them down on a piece of paper. Six signiﬁcant ﬁgures is good! It helps to zoom in a lot (as shown in the above example) to get the most accurate reading.

3. Return to GMDE and choose Settings>Set Coordinate System Offset. A dialog box will appear instructing you to click on a very well deﬁned point, that is the same one that you got the coordinates for in Google Earth.

4. Click on that point and then in the same dialog box, two text boxes will appear where you can enter the same longitude and latitude that you wrote down in Google Earth. Click Okay.

5. A second dialog box will appear telling you the exact offsets that GMDE calculated for your map, usually on the order of hundreds of feet or tens of meters.

Once you have set the coordinate system offset, if you hover the mouse over a corner of the map that you entered in the georeferencing part, the latitude and longitude that appear in the location box in the lower left corner of the map will no longer be what you entered but some- thing slightly different! Don't worry! GMDE is now showing you the coordinates of the point in WGS84. GMDEs ad- justment is approximate because it is constant over the entire map. None- theless it should be close enough assuming your map area isn’t too large.

At any time, you can see the metadata for your map that GMDE has calculated by selecting Settings>Show Map Metada- ta. An example is shown to the right.

Digitize Existing Strike and Dip Symbols

GMDE makes it (relatively) easy to extract orientation data from a scanned map. To do so, proceed as follows:

1. Select Settings>Mode>Strike & Dip Only. Make sure that “user entered strike and dip checkbox is checked in the tab panel on the right

2. To capture a strike line, click the “click” button to the left of the word “Strike” or select Operations>Drag Strike Line (⌘G).

3. Click the mouse at the center point of the scanned strike line (which is usually where it intersects the dip tick mark) and drag the mouse in the direction of the right hand rule strike. The line will expand out symmetrically in both directions from the center

point. You can drag as far as you want and, in fact, the farther you drag, the more accurate your measurement will be.

4. GMDE will display a dip tick mark oriented correctly (i.e., using the right hand rule) for the direction in which you are dragging. It will also display an interactive dip value that will increase in number the longer you make the strike line (see diagram, below).

5. Once you release the mouse button, GMDE will automatically record both the strike and the coordinates of the measurement in the panel to the right, as shown above.

6. GMDE will also paste the interactive dip from step 4 in the dip text box. The contents of that text box will be selected so that you can type in the dip value if the interactive- ly determined dip was not correct.

7. If you did not drag in the direction of the right hand rule strike, you can correct it by selection the quadrant direction of the dip from the popup menu just above the dip text box. The interactive display of the dip tick mark makes this unlikely.

8. When your are satisﬁed with the measurement, click the “Record S & D” button. Your measurement is not saved until you do this step!

Calculate Orientations from 3-Point Problems

The orientation of any planar feature can be determined from any three points on its surface so long as at least one of the three points is at a different elevation than the others. GMDE makes this task especially easy by allowing you simply to click on the three points. Proceed as follows:

1. Locate a stratigraphic contact or other planar feature where it crosses topography by “V-ing” across ridges and valleys. Make sure that the “user entered strike and dip” checkbox is NOT checked.

2. To click on the ﬁrst point, there are two different methods:

• More straightforward: Press the “click” button located next to the top row of coordinates in the “Surface 1 3 Pt Calculation” group box and then click the mouse on the geological contact trace on the map.

• Faster: position the mouse in the map over the point on the surface that you want to measure. Hold down the Shift Key while you right click on the point and select “Click Point 1” from the contextual menu. This requires just a single click to enter the XYZ coordinates of the point.

3. Repeat step 2 for the other two points, using the appropriate “Click” button or choice from the contextual menu.

4. GMDE will then automatically calculate the strike and dip of the surface.

5. When you are satisﬁed with the measurement, click the “Record Strike & Dip” button. Your measurement is not saved until you do this step!

Don’t be surprised if the orientation from the 3 point problem comes out differently than geologist measured strikes and dips, as shown above! There are many potential expla- nations for the discrepancy and the discrepancy itself can be an important observation!

Select a Subset of Strikes and Dips for Analysis Using Lasso

Commonly, you only want to use a subset of your strikes and dips on a map for an analysis such as calculating a cylindrical fold axis, etc. You could do this the hard way by clicking the checkbox by each observation in the listbox at the bottom of the right hand panel, but there is an easier way:

1. Enter a series of strikes and dips, either by reading them in from a disk ﬁle or drawing them on the map by dragging the strike line

2. Zoom the map out so that you see the entire group of strike and dip symbols that you want.

3. Select Edit>Lasso Select>Lasso Inside (⌘L). The cursor will not change.

4. In the map window, drag a line all of the way around the data that you want to select. GMDE closes the polygon automatically so that you can tell for sure which observations are inside the lasso. When you are ﬁnished, release the mouse button.

5. GMDE will select all of the observations inside the lasso, giving them a yellow halo in the map view and selecting them in the listbox to the right.

6. Select Edit>Toggle Off>Not Selected in List. All of the observations except for the selected ones will be unchecked and their strike and dip symbols will disappear from the map.

7. Any subsequent analysis involving strikes and dips will only include the remaining checked observations.

Note that Lasso Select also works for selecting a group of vertices along a selected contact or along a selected line of section. Combined with the Toggle On and Toggle Off commands, it enables a lot of capabilities.

Measure Stratigraphic Map Thicknesses from 3-Point Problem

GMDE can determine the thickness of a stratigraphic unit from one know point on the base and another known point on the top of the unit as well as the orientation of the unit. Like all such calculations, we have to assume that the base and top of the unit are parallel. It is most convenient to determine thickness where you have measured the orientation from a three point problem because you already have points on one of the bounding surfaces of the formation.

1. If you haven’t done so already, select Settings>Mode>Strike Dip Thickness. A group box for entering the coordinates from the second surface will appear in the panel to the right.

2. Click on a point on the second surface, using either of the procedures in step 2 of the previous section of this document. The easiest is to use the contextual menu with the Shift Key. If you use that, select “Click Point 4” from that menu. The point on the second surface does not need to be down dip from the point on the ﬁrst surface.

3. Assuming you already have done a three point problem, you will need to click the radio button next to the point out of the three on the ﬁrst surface that you want to use in the calculation of the thickness. GMDE will display the calculated thickness in the Surface 2 group box.

4. In order to save your measurement, you must click the “Record Thickness” button. Otherwise whatever you have entered will be overwritten.

Measure Stratigraphic Map Thicknesses using mapped strike and dip

You can also measure stratigraphic thickness using a strike and dip already on the geology map. Proceed as follows:

1. Select Operations>Drag Strike Line (⌘D) and drag the strike line as you normally would, but do NOT press the “Record S & D” button.

2. Use the “Click” button one the ﬁrst row of surface 1, or use the equivalent right- mouse click contextual menu item, or use Operations>Add>Surface 1 Pt 1 (⌘1), to add surface 1 point 1 on one of the two contacts of the formation that you want to

determine the thickness. The dot where you dragged the strike line will disappear but don’t worry, the dip is still recorded in the panel to the right.

3. Set surface point 2 — using the contextual menu, menu bar equivalent (⌘4), or “Click” button in the surface 2 group box — on the opposite contact of the formation. E.g., if you set surface 1 pt 1 on the base of the formation, you would set surface 2 on the top formation contact. The two points do not have to be up or down dip from one another.

4. The thickness will be displayed in small print at the bottom of the Surface 2 group box in the panel to the right. If everything is as you would like, click the “Record Thickness” button. Your measurement is not saved until you do so

5. Surface 1 Pt 1, on the base or top of the formation, will be assigned the strike and dip that you measured in step 1 (because you are assuming that the entire formation has that orientation), even though the mapped orientation symbol is quite unlikely to have been located on the contact (see the above ﬁgure).

Propagate Errors in Your Analysis

All measurements have errors associated with them and those errors should propagate through the calculations to yield uncertainties on the calculated parameters. The previous calculations can propagate errors to determine the uncertainty of orientation and map thickness.

1. In the “Orient & thickness” tab of the right hand panel, go to the “Set Er- rors” group box.

2. The numbers that you enter in the Horizontal and Vertical text boxes as- sign uncertainties to the spatial de- termination of the each point that you click. These errors then ﬂow into three point and map thickness calculations. The horizontal error is displayed on the map as a circle outline around each point.

3. The Strike and Dip errors are applied when the strike and dip is determined by dragging a line. You can only enter values here when “user entered strike & dip” is checked.

4. Errors are only applied at the time of calculation. Once a datum has been stored, you cannot retroactively apply errors.

You may well be surprised at how large the propagated errors are. This just emphasizes how we commonly underestimate uncertainty in our results. If you enter statistical uncertainties on the input parameters, then you will get statistical errors out on the propa- gation. Statistical errors are determined by repeated measurement and in most cases you are unlikely to do that (i.e, you are unlike to go out to the map areas with a hundred geologists and ﬁgure out how much variation there is in where they locate the contact!). Without statistical errors, you are just stuck with guesstimates which may be better than nothing but may also be misleading.

Digitize Contacts

Digitizing contacts on a paper map is extremely tedious but GMDE makes this chore tolerable if not easy. Nonetheless, contacts digitized in XYZ are essential for doing true 3D down-plunge proﬁles, plotting in Google Earth, etc. Because GMDE gets the elevation of each point from the elevation servers on the Internet, this task consists of nothing more than clicking along a contact at relatively even intervals:

1. Select Settings>Mode>Digitize Contacts. This mode is set automatically if you select the Contacts tab of the right hand panel or select Operations>Draw New Contact.

2. To start drawing a new contact, select Operations>Draw New Contact (⌘K).

3. Navigate to where you want to begin digitizing the contact. Click and release the mouse on the ﬁrst point.

4. Move to the next point and click and release. GMDE will mark each point with a yellow square and will draw a straight line between the vertices. It will acquire an elevation for each vertex. You will probably want more closely spaced vertices where the contact is highly curved and more distance between vertices where contacts are relatively straight.

5. Repeat clicking and releasing the mouse to set each vertex of the polygon until you have completely digitized the surface. If your line wanders from the contact, don’t worry. You can move individual vertices later.

6. To end a contact, double click the mouse button without moving the mouse. Alterna- tively, you can click the “Finished” button in the small ﬂoating window that indicates that digitization is in process. GMDE will ask you to name the contact and will then add it to the contacts list box.

7. The upper list box in the Contacts tab shows all of the contacts that have been entered whereas the lower list box shows the individual vertices for the selected contact. To turn off an entire contact, uncheck it in the upper list box. To turn off individual vertices uncheck them in the selected contact vertices list box. You can turn of a group of vertices as once by selecting all of them in the list box (noncontiguous selection can be accomplished with the command key), and then using the Toggle

commands in the Edit menu. As shown below, selected vertices in the listbox are highlighted with yellow halos in the map view.

When a contact is selected in the contact list box in the upper part of the Contacts tab panel, the vertices are plotted as small squares along the contact in the map view. You can click and drag a vertex in the map view to reposition without issuing a special command. Once you reposition the vertex, GMDE will retrieve the elevation at the newly po- sitioned point. If for some reason the elevation server is down or responding slowly, you can come back at a later time and get the elevations by selecting Operations>Get Ele- vations for Selected Contact. The map length and map area of the selected contact is shown between the two contact list boxes. Thus, you can use drawing contacts to measure the area of any feature by clicking the mouse around its perimeter.

Construct a Down-Plunge Fold Profile

Cylindrical folds are those in which the geometry of the fold does not vary in a direction parallel to its axis. Thus, points along bedding surfaces at different positions can be projected parallel to the axis onto a two dimensional proﬁle plane which provides the truest view of the fold geometry. For plunging folds, the proﬁle plane is non-vertical (but see next section). In reality, very few if any natural folds are truly cylindrical but the approxi- mation is still useful where you have limited data.

1. If you haven’t already done so, digitize some contacts on both ﬂanks of a plunging fold.

2. Determine the fold axis which is best done by calculating a cylindrical best ﬁt to bedding orientations. This can be done in a Stereonet program but, if you have already digitized strikes and dips in GMDE, you can select Operations>Calculate Cylindrical Fold Axis to get the trend and plunge of the axis as shown in the dialog box above.

3. Select Plot>Down Plunge Projection. You will be asked to enter the fold axis that you determined in step 2.

4. The Down Plunge Projection window will appear showing a proﬁle view of the fold. As shown below, this window is linked to the map view: select a vertex on a contact on the proﬁle and the same vertex on the map is also highlighted with a yellow hal- low. The colors of the beds in this view are controlled from the Inspector palette.

5. You can incrementally adjust the fold axis trend and plunge with the controls in the lower right and watch the down plunge projection change in real time. You can use this feature to search for the best ﬁtting fold axis where you have very limited infor- mation.

6. If you want to save a vector graphic of your down plunge projection, press the “Save as SVG” button. SVG (for Scaleable Vector Graphics) ﬁles can be read by most modern vector drawing program and display at high resolution in web browsers.

Down plunge projections can be useful for identifying potential mapping errors. For example, in the below ﬁgure the sudden offset of the beds on the forelimb on the right hand side is probably due to a mismapped fault.

Get the Elevation of a Cylindrically Folded Surface at Depth

The down-plunge proﬁle described above usually is not a vertical plane but is inclined. There are times when you want to know how deep a cylindrically folded surface is beneath a point on the surface, perhaps because you want to drill to a particular horizon or are drawing a vertical cross section.

1. Just as in the previous example, you will need to have digitized some beds and will need to know the trend and plunge of the fold axis.

2. Choose Operations>Depth to Folded Surface>For Selected Surface (⌘J). Click the mouse where you want to know the surface.

3. A dialog box will appear asking you to enter the trend and plunge of the fold axis. Enter the values and click Okay.

4. A dialog box (below) will appear showing the stratigraphic surface, position of the clicked point, and the elevation of the surface at that point. GMDE also marks the point on the map and displays the elevation of the surface beneath that point with white text as shown.

5. If you want to measure subsequent depths without having to enter the fold trend and plunge dialog every time, after choosing …For Selected Surface (⌘J), hold down with Shift Key while clicking on the map. GMDE will assume you want to use the same fold axis as the last time

6. The values in the dialog box table can be copied and pasted into a spreadsheet or other program. You can also save a text ﬁle of the same data.

Construct Topographic Profiles with Contacts and Apparent Dips

One of the most tedious tasks that every new structural geology student learns is constructing topographic profiles and plotting apparent dips and contacts accurately on those profiles in preparation for drawing a cross section. Well, no more! GMDE can construct topographic profiles with any number of bends in them. To have the program plot apparent dips and contacts, you will need to have already entered those in the program by digitizing contacts and strike and dip symbols (or using three point problems).

1. Decide where you want your line of section to be located, which will depend on your purpose in drawing the section. Your section may be a single straight line or may have any number of bends. You can, for example, use this tool for constructing stream proﬁles by clicking along the trace of the stream.

2. Make sure that GMDE is set to use the elevation server that you want and that the sampling of elevations along the profile has the spacing that you want. Both of these are set in the inspector palette under the Topo Profile tab. You set the sample spacing by specifying the number of pixels between elevation values. In the figure to the right, the sample spacing is 10 pixels; because

the map resolution is 6.6 ft/pixel (2 m/pixel), that means that GMDE will get an elevation from the server every 66 ft (20 m) along the proﬁle. The smaller the sample spacing, the longer it will take to retrieve the elevation values from the server.

3. Select Operations>Define Topo Profile (⌘T) to start the process of defining the topographic profile on the map. A small floating window will appear reminding you that you are in the process of drawing a line of section.

4. Click and release the mouse button at the start of the profile. Then click and release the mouse anywhere you want a bend in the section or you would like to make a custom annotation. Each time you click and release the mouse, GMDE will automatically select the notes field next to the clicked coordinate in the list box on the right hand side, ready for you to type in your annotation. Annotations will appear on your profile just as you type them.

5. To finish defining the topographic profile, double click the mouse at the end of the section or press the “Finished” button in the small floating window. The floating window will disappear and a new window will appear asking you to name your profile. You can always change the name later if you want.

6. Once you have named the profile (or just accepted the default name), GMDE will request the elevations from the specified server at the specified spacing. This generally takes longer with the USGS server because each elevation is a separate request whereas the MapQuest server can send an entire profile with one request. In either case, retrieving the elevations can take as long as minute or two. GMDE will display a progress bar while it is in the process of getting the elevations. Be patient; as slow as the servers may be, it is still much faster than doing it by hand! You can do other things in the program or in other programs while GMDE is querying the server.

7. To plot your topo proﬁle, select Plot>Topo Cross Section and you will be presented with the dialog box to the right where you will specify what gets plotted on the topo proﬁle. Note that the projection azimuth of strike and dip measurements lying off section should be equal to the fold axis trend and not parallel to strike (except where the folds have zero plunge in which case the two are the same!). This box provides considerable granularity in terms of what to plot, including contacts projected onto the vertical section using a cylindrical fold geometry.

8. Once you click “Okay” in the dialog box, the topographic proﬁle window (shown below) will appear with your proﬁle and data, including cylindrically folded contacts, if

selected, plotted on it. The proﬁle will shrink or expand as you resize this window. It may well be that the contacts are projected from too far a distance. I that is the case, you can use the lasso, described in a previous section, to select and turn off bedding vertices.

9. Occasionally, a server will return a bad elevation which will appear as a large negative spike on your profile. If this happens, locate the bad value in the profile vertex listbox to the right of the map and uncheck the value. The profile will be redrawn with a gap where the bad value was.

10.The opacity of the apparent dip tick marks varies with distance from the proﬁle: the fainter the tick mark, the farther off the line of section the measurement. Any over- turned beds are plotted in a different color so you can identify them. You can change the vertical exaggeration of your plot with the pop-down menu in the lower right; the apparent dip tick marks will change correctly with vertical exaggeration.

11. The topo proﬁle is linked to the map: select a vertex on the proﬁle and the same vertex on the map will be selected with a yellow halo and vice versa.

12.Finally, you can save the proﬁle and axes as a vector graphic by clicking “Save as SVG”. This will save exactly what is showing in the window at the current scale of the window.

Plot Topo Profile Along Part of a Line of Section, only

There may be times when you want to plot part of a lines of section only without having to deﬁne a whole new line of section and wait for the elevations to download all over again. You might also want to construct your section piecemeal if different parts of your section have differently oriented fold axes. You can easily accomplish this task in GMDE:

1. Select the proﬁle line that you want to plot just part of.

2. Select the vertices of that proﬁle that will constitute the active line of section. There are two possible ways of accomplishing this: (a) Click on the starting vertex in the listbox, then click on the ending vertex while holding down the Shift Key, or (b) Se- lect Edit>LassoSelect>Inside Lasso and then drag the mouse around the part of the section line that you want to select.

3. Select Edit>Toggle Off>Not Selected in List. The unselected vertices along the line of section will be unchecked in the vertices listbox.

4. Now, when you choose Plot>Topo Cross Section only the checked vertices will be plotted.

Note that if you uncheck vertices in the middle of the line of section, but leave the ends checked, the entire section will still be drawn but the topography will not be shown for the unchecked vertices. For this to work as expected, you want a single, contiguous group of vertices.

Calculate Piercing Points on a Fault or other Planar Surface

To calculate piercing points, you ﬁrst need to identify on a map a fault that where you know the orientation of the fault plane somewhere on the ground, and can locate the same linear feature with known trend and plunge on either side of the fault plane. Linear features that you might want to use include fold axes on a particular stratigraphic inter- face, the intersection between a dike and a known stratigraphic horizon, truncation of a stratigraphic horizon beneath an angular unconformity, etc. By the way, this same calculation can also be used to determine how far you have to drill at a particular angle to in- tersect a buried planar feature of interest, etc. Once you have located the planar and linear features, proceed as follows:

1. Select Operations>Get Piercing Points (⌘Y). The Piercing Pts tab should appear on the right hand side if it is not already showing.

2. Click on a known point on the plane whose orientation you already know. There are two ways to do this:

• More straightforward: Press the click button next to the “Plane Intersected” group box and then click the mouse on the fault trace on the map.

• Faster: position the mouse in the map over the point on the fault trace that you want to click. Hold down the Shift Key while you right click on the point and select “Click point on Plane” from the contextual menu. This requires just a single click to enter the XYZ coordinates of the point.

3. Type the strike and dip of the plane using right-hand rule into the boxes just below the coordinate boxes

4. Click on the map where a line of known orientation outcrops. To accomplish this, you have the same two ways of doing it as in step 2.

5. Type in the trend and plunge of the line in its group box.

6. Click on the same geologic line located on the other side of the fault using one of the same two methods as described in step 2.

7. GMDE will helpfully ﬁll in the same trend and plunge as you used for the ﬁrst line but you can change that to whatever you want

8. When you are happy with your analysis, click the Record Piercing Points button. This will enable you to see the results plotted on the map and preserves them for later access.

On the map, GMDE shows (a) the strike and dip of the fault plane that you entered in step 3, located at the place you clicked in step 2; (b) triangles at the points of the two lines that you clicked; (c) two target like symbols that show the projection of the two piercing points on the map. Each target is accompanied by an elevation number which will, in all probability, be located under or above the ground. GMDE calculates the slip vector magnitude and orientation from the XYZ positions of the two piercing points. The map display can be confusing, so GMDE also offers the opportunity to visualize the fault plane and slip vector in a stereonet by choosing Operations>Stereonet of Slip on Fault. The below ﬁgure shows how we have used piercing points to determine the slip on an oblique fault in the Canadian Rockies thrust belt using the synclinal axis at the base of the –Cpk unit.