Let's see where exactly it is
Mostly you see the drawings like above for different chakras. Above you see the Agya (Ajna or 3rd Eye) Chakra and it has Trikuti - Guru Chakra on top of it. Means Guru Chakra - Trikuti (Triangle) is overlapping the Center Circle of Agya Chakra.
Note: Pronunciation is "Agya" but it is written as "Ajna" due to some Translation Mahabharata. Just like pronunciation is Gyana or Gyan but it is written as "Jnana". Agya Chakra is also called as 3rd Eye Chakra, because of one of its expression as "seeing beyond" what is visible through other two eyes. Or in other words, it represents the "Seer of Truth" which means having Vivek (Power of Discrimination). Note: In the above Diagram 2 left petals and a circle in between is Agya Chakra. The Triangle in between the inner circle is Trikuti - Guru Chakra and the OM Symbol is the representation of Chakras beyond Sahasrara (Crown Chakra). This diagram is like a layered representation, in fact, a system called as "Brahma-Granthi" (Knot of Supreme Consciousness or Divine Cosmos).
While looking at these drawings you forget a few things. Let's see what confusions it creates.
Two Dimensional Representation
These drawings are 2 dimensional. Means you are only seeing the shape in terms of length and breath only. Depth is missing from all paper drawings. In 2 dimensional drawings if I ask you to draw one line on top of another, then to the reader they will look like one line only. If I ask you to draw X on top of Y, then for the reader there is no way to know whether X is on top of Y or Y is on top of X, because depth cannot be shown on paper, in the 2-dimensional drawing. So what is the way for me to show you that X is above or on top of Y? The best way is to draw it "vertically up above" the Y. If I have to show you that there are two lines on top of each other, then I have to draw one line vertically up above the first line. "View Point" is also important. If one is looking from the side (Horizontal, Left to Right & vice-versa) then objects are drawn differently than when View Point of the Reader/Viewer is from the Top (Vertically, Top to Bottom and vice-versa). Please look at the below picture carefully.
You also need to understand that English Language or any language for that matter is also mostly two dimensional in nature. Without using appropriate words it's not easy to explain that which has the depth or multiple dimensions. When we say something is on Top of another thing, it could be horizontally up above or vertically up above. Overlapping is the word to use when we are talking about depth but if our drawing is two dimensional then it might counteract the fact that we are trying to explain.
You are a multi-dimensional being, your chakras are multidimensional too. Chakras are NOT fixed pathways, means they are not like the built roads in the city, instead they are instant, dynamic, created on demand kind of pathways of a forest. Entire Pranamaya Kosha is like that. The way energy moves, that many pathways are created. In total there can be 72,000 (Nadis) pathways that can exist. Energy is constantly flowing and rotating in specific patterns we call chakras. Also, know that your Pranamaya Kosha is bigger than your physical body. See the below picture to see different colours that energy of different frequencies will create, rotation pattern of energy and the way it goes on top and sideways on your forehead. See the two side petals of the Agya Chakra in the energy rotation pattern. This picture is for reference purpose only, it should not be used to baseline colours, patterns or exact size of Agya or Trikuti. Because different people have different strength of Pranic Energy and different Vasanas in Manomaya Kosha that reflects as Blockages in different chakras. Two Dimensional drawings of all the Chakras were made using those people who have had those Chakras OPEN (without any blockage) in them. Those who have closed or blocked Chakras their own Chakra drawing will create incomplete Chakra on paper. If someone's Trikuti is not open then that means the Triangle is "missing". If Agya chakra is closed that means the entire drawing is missing, because those lines, circles, triangles, petals are nothing but Energy/Pranic Shakti pathways. If Shakti is not flowing in them they are invisible or non-existent.
There is a whole lot of "depth" in this chakra especially and this is HIGHLY Multi-Dimensional Chakra in comparison to other Chakras because it involves Cosmic Dimensions and Chakras as Shri Bindu, Ardh-Chandra Bindu, Shri Atma-Paramatma, Shri Adishakti and Sri Sadashiva (These 5 are represented as OM Symbol in the center of Agya Chakra and these are ABOVE Sahasrara Chakra).
Not just vertically up above, but beyond the existence of Agya. These are not personal or individual to anybody, they are Universal Existential Centers.
Here is another Multi-Dimensional looking Drawing of Agya Chakra which includes Guru Chakra & Other 5 Cosmic Existential Centers and the Linga. A complete drawing of "Brahma-Granthi".
Guru Chakra or Trikuti is "overlapping" the inner circle of Agya Chakra. Or we can say that Guru Chakra or Trikuti is on top of the Agya Chakra. When we say it's "on top" that doesn't mean it is Vertically up above, instead it means in terms of depth Trikuti is on top of Agya. Two Dimensional drawing on paper doesn't include the third dimension of depth hence to show "overlapping" object are often drawn "vertically up or down".
The generations that exist today mostly have studied "Euclidean Geometry" which was only about the 2-dimensional world and taught us that " if you add all 3 angles of a triangle then it will not exceed 180 degrees". A century later when another person drew the Triangle on the Globe (3D Sphere) then it was concluded that Triangle's 3 angles can be more than 180 degrees if 3rd Dimension is added. That's how Flight Paths for Aeroplanes are created/managed. Depth in the form of Longitude has to be considered. Even though all this was discovered a century ago still schools and education system did not include it in the syllabus and we Multi-Dimensional beings just kept thinking in terms of two or at max 3 dimensions only. Our multi-dimensional nature was removed from the education system.
Advice for Spiritual Seekers
You know "water" is called as "Paani" in Hindi, "Jal or Jalam" in Sanskrit. Its chemical formula is H2O. Water can be found directly in Rivers, Wells, Gorges, Glaciers and Oceans. Not all water is drinkable. Water can be found in other forms in Ice, Clouds, Earth, Trees and so on. I can go on and on and on about Water. But will it quench your thirst? No!
You cannot know about "any" of your Chakras "directly" until your Agya Chakra is Open. Means you may have an open Mooladhara or open Swadhisthana Chakra or any other Chakra may be open or close in you, but you will NOT know it directly through your Pranamaya Kosha. You may know that you do not have fears of any kind and hence your Mooladhara is open, but there is no way to "know" or "perceive" the presence of it in Pranamaya Kosha directly until your Agya Chakra is Open.
When your Agya is open you can perceive your Chakras, Lights, Sounds, Shapes, Energy associated with them directly in your Pranamaya Kosha. Although I am not sure "what help" it can provide? If I can see the anatomy of my nervous system, I may still be not able to remove the blockages until I know the connection of nerves with muscles and organs and other systems/parts of my body. And in order to "know" the entire system the only way is to become 100% Conscious, means Enlightened. So those who can see/feel Shapes, Lights, Sounds, Energy of different Chakras I am not sure if it is of any good to them or anyone else. As a spiritual seeker, you need not get into all this knowledge and learning and waste time because your ultimate aim is to go beyond Light and Sound and Form. Focus more on removing Vasanas and not entertaining yourself with the Light & Sound Show!
Agya can show you the Chakras and their Mental/Emotions and Physical manifestation, but you will NOT know the true "purpose" or "the way" to purify each Chakra, unblock them until your Trikuti - Guru Chakra is Open or you find a Real Guru. Trikuti or Real Guru is needed, the Seer of Truth, the Vivek is needed to find the ultimate manifestation, expression of each Chakra and realize it's full potential.
As a spiritual seeker, understand that knowing the exact location of your TRIKUTI, or it's anatomy or physiology is not going to OPEN YOUR TRIKUTI. Only specific meditations, a substantial amount of Vasana release, less karmic load, breached or full Enlightenment/Nirvikalpa and can open your Guru Chakra - Trikuti. Kundalini has to shoot all the way up above your head technically to open Trikuti and keep it open. Hence focus more on opening it than reading/learning/gathering bookish knowledge about it. Trikuti Meditation is different than Agya Meditation, though the centre of forehead seems like the place for both. If your Guru has given you Trikuti Meditation, then know that you have to open it, you can open it and you are ready to do that. I will forever be in gratitude that my Trikuti opened first before I read about it, else who knows, I might have had similar confusion about its position.
I bow down to the Guru who is the Seer within me, who is also the Seen and can help others See the Truth through me.
गुरुर्ब्रह्मा गुरुर्विष्णुर्गुरुर्देवो महेश्वरः । गुरुरेव परं ब्रह्म तस्मै श्रीगुरवे नमः ॥१॥
Meaning: 1.1: The Guru is Brahma, the Guru is Vishnu, the Guru Deva is Maheswara (Shiva), 1.2: The Guru is Verily the Para-Brahman (Supreme Brahman); Salutations to that Guru.
अखण्डमण्डलाकारं व्याप्तं येन चराचरम् । तत्पदं दर्शितं येन तस्मै श्रीगुरवे नमः ॥२॥
Meaning: 2.1: (Salutations to the Guru) Whose Form is an Indivisible, Whole of Presence, and By Whom is Pervaded the Moving and the Non-Moving Beings, 2.2: By Whom is Revealed (out of Grace) That Feet (of Indivisible Presence); Salutations to that Guru.
अज्ञानतिमिरान्धस्य ज्ञानाञ्जनशालाकया । चक्षुरुन्मीलितं येन तस्मै श्रीगुरवे नमः ॥३॥
Meaning: 3.1: (Salutations to the Guru) Who Removes the Darkness of Ignorance from our Blind (Inner) Eyes by applying the Collyrium of the Light of Knowledge. 3.2: By Whom our (Inner) Eyes are Opened; Salutations to that Guru.
स्थावरं जङ्गमं व्याप्तं येन कृत्स्नं चराचरम् । तत्पदं दर्शितं येन तस्मै श्रीगुरवे नमः ॥४॥
Meaning: 4.1: (Salutations to the Guru) By Whom is Pervaded all the Movable and Immovable objects as well as the Moving and Non-Moving beings, 4.2: By Whom is Revealed (out of Grace) That Feet (of All-Pervasive Presence); Salutations to that Guru.
चिद्रूपेण परिव्याप्तं त्रैलोक्यं सचराचरम् । तत्पदं दर्शितं येन तस्मै श्रीगुरवे नमः ॥५॥
Meaning: 5.1: (Salutations to the Guru) Whose Form is that of Consciousness Pervading all the Moving and the Non-Moving beings of the Three Worlds, 5.2: By Whom is Revealed (out of Grace) That Feet (of Conscious All-Pervasive Presence); Salutations to that Guru.
सर्वश्रुतिशिरोरत्नसमुद्भासितमूर्तये । वेदान्ताम्बूजसूर्याय तस्मै श्रीगुरवे नमः ॥६॥
Meaning: 6.1: (Salutations to the Guru) Who is the Embodiment of All Shrutis (Vedanta) which Equally Shine (He being the Essence of them) like Jewel worn on the Head, 6.2: Who is the Sun blossoming the Lotus of Vedanta. Salutations to that Guru.
चैतन्यः शाश्वतः शान्तो व्योमातीतोनिरञ्जनः । बिन्दूनादकलातीतस्तस्मै श्रीगुरवे नमः ॥७॥
Meaning: 7.1: (Salutations to the Guru) Who is the Eternally Tranquil Consciousness, Spotless and Pure, and Beyond the Ether, 7.2: Who is Beyond the Bindu, Nada and Kala; Salutations to that Guru.
ज्ञानशक्तिसमारूढस्तत्त्वमालाविभूषितः । भुक्तिमुक्तिप्रदाता च तस्मै श्रीगुरवे नमः ॥८॥
Meaning: 8.1: (Salutations to the Guru) Who is Equally Mounted on Jnana (Knowledge) and Shakti (Power), and Who is Adorned with the Garland of Tattva (Truth or Absolute Reality), 8.2: Who Grants both Worldly Prosperity and Liberation; Salutations to that Guru.
अनेकजन्मसम्प्राप्तकर्मेन्धनविदाहिने । आत्मञ्जानाग्निदानेन तस्मै श्रीगुरवे नमः ॥९॥
Meaning: 9.1: (Salutations to the Guru) Who Burns away the Fuel of Karma (results of works impressed on the mind) Accumulated over Many Births, 9.2: By Giving (Kindling) the Fire of Self-Knowledge; Salutations to that Guru.
शोषणं भवसिन्धोश्च प्रापणं सारसम्पदः । यस्य पादोदकं सम्यक् तस्मै श्रीगुरवे नमः ॥१०॥
Meaning: 10.1: (Salutations to the Guru) Who Dries Up the Ocean of Samsara (Worldly Existence) and Leads to the Essential (Spiritual) Wealth Within us, 10.2: In the Same Manner as His Foot-Water (i.e. grace, when a devotee surrenders everything at His Feet) removes the impressions of the Samsara from the devotee's mind and reveals the Essential (Spiritual) Wealth Within; Salutations to that Guru.
न गुरोरधिकं तत्त्वं न गुरोरधिकं तपः । तत्त्वज्ञानात् परं नास्ति तस्मै श्रीगुरवे नमः ॥११॥
Meaning: 11.1: (Salutations to the Guru) Neither is there any Reality Beyond the Guru, Nor is there any Austerity Higher than the Guru, 11.2: There is no Knowledge of Truth beyond what comes From the Guru; Salutations to that Guru.
मन्नाथः श्रीजगन्नाथो मद्गुरुः श्रीजगद्गुरुः । मदात्मा सर्वभूतात्मा तस्मै श्रीगुरवे नमः ॥१२॥
Meaning: 12.1: (Salutations to the Guru) My Lord is the Lord of the Universe, My Guru is the Guru of the Universe, 12.2: My Self is the Self of All Beings; Salutations to that Guru.
गुरुरादिरनादिश्च गुरुः परमदैवतम् । गुरोः परतरं नास्ति तस्मै श्रीगुरवे नमः ॥१३॥
Meaning: 13.1: (Salutations to the Guru) There is no Reality which existed Before the Guru And the Guru is the Supreme Divinity, 13.2: There is no Reality Surpassing the Guru; Salutations to the Guru.
ब्रह्मानन्दं परमसुखदं केवलं ज्ञानमूर्तिम् द्वन्द्वातीतं गगनसदृशं तत्त्वमस्यादिलक्ष्यम् । एकं नित्यं विमलमचलं सर्वधीसाक्षीभूतम् भावातीतं त्रिगुणरहितं सद्गुरुंतं नमामि ॥१४॥
Meaning: 14.1: (Salutations to the Sad-Guru) Who is the Bliss of Brahman, Who is the Bestower of Supreme Joy, Who is the Absolute, Who is the Embodiment of Knowledge,
14.2: Who is Beyond Duality, Who is Boundless and Infinite Like the Sky, Who is Indicated by Maha Vakyas Like Tat-Tvam-Asi (That-Thou-Art).
14.3: Who is One without the Second, Who is Eternal, Who is Stainless and Pure, Who is Immovable, Who is the Witness of the Intelligence of All Beings,
14.4: Who is Beyond the States of the Mind, Who is Free from the Three Gunas; Salutations to that Sad-Guru.